'Forager' was the US seizure of the three largest islands of the Mariana islands group within the overall 'Granite' and 'Granite II' plans (15 June/10 August 1944).

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During the Pacific and South-West Asian campaigns of 1943 and the first half of 1944, the Allies had captured the Solomon island, Gilbert island and Marshall island groups as well as the Papuan peninsula of New Guinea. In the Pacific this brought the Americans up against the main Japanese defence line, which was based on the Palau, Caroline, Palau and Mariana island groups, ex-German possessions occupied by the Japanese since the end of World War I and heavily fortified in the period between the two world wars.

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The islands of the Marianas group lie in an arc about 1,400 miles (2250 km) to the south of Japan. The group comprises 15 islands extending about 425 miles (685 km) from Farallon de Pajaros in the north to Guam in the south. Most of the islands are mountainous, with peaks up to 2,585 ft (788 m) in height, and the total land area of the group is 388 sq miles 1005 km²), of which more than half (210 sq miles/544 km²) is Guam, the single largest land mass of the whole group. The islands are characterised by relatively fertile soil, and are covered with mixed scrub and grassland, with a few mangrove swamps. Their beaches are generally narrow, backed by coral cliffs and with reefs off many parts of their shore lines.

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Located in the deep Pacific ocean basin, the islands have no sedimentary basement complex and no interesting mineral resources, but possess climate eminently suitable for the cultivation of sugar cane, and as such the islands supplied much of Japan’s sugar. So far as World War III was concerned, moreover, they were also strategically located along the sea lanes to Japan’s mandates and also to the eastern part of New Guinea and the island groups to this huge island’s east and south-east.

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The Mariana islands group was a Spanish colony from 1667, and remained under the general government of the Spanish administration of the Philippine islands group until 1898, when, as a result of the Spanish-American War, Spain ceded Guam to the USA. By a treaty of 12 February 1899, Spain sold the other Spanish-held islands of the Mariana group to Germany, and together with some 6,000 islands in other island groups, such as the Caroline and Pelew island groups, all formerly under Spanish control but now indefensible by Spain, the islands were now incorporated as a small part of the larger German Protectorate of New Guinea.

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As a member of the Triple Entente, Japan began to occupy the northern part of the Mariana islands group in 1914. After Germany and the rest of the Central Powers lost World War I, many formerly German-controlled islands in the Pacific were entrusted by the League of Nations to Japanese control as the South Pacific Mandate. These islands included the northern islands of the Mariana group, which were developed for sugar production under the auspices of the South Seas Development Company. By this time the Japanese population of the islands outnumbered the indigenous population (Chamorros) by two to one. Rota, just to the north of Guam, produced enough sugar to support two refineries and a distillery, and had a population of 764 Chamorros and 4,800 Japanese and Koreans. A small airstrip was constructed on the northern part of the island during the war. To the north of Rota lie the group’s two other important islands, Tinian and, to its north, Saipan.

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The naval disarmament treaties of the inter-war years had specified that these islands were not to be fortified, but with the lapse of the treaties in the early 1930s Japan began to construct large airfields on Saipan, within easy range of Guam. The USA did not build fortifications on Guam, which was considered too exposed to be held in the event of war. Japanese troops landed on Guam just two days after the 'Ai' attack on Pearl Harbor and easily conquered the island.

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The Mariana islands group had been identified as an important objective in the 'War Plan Orange' inter-war plan outlining the USA’s military objectives in the event of conflict with Japan, but it was not until August 1943, at the 'Quadrant' conference in Quebec, that a formal decision was made to capture the Mariana islands group following the seizure of the Palau islands Group. In December 1943, at the 'Sextant' conference in Cairo, the US Army Air Forces forcefully argued for an early invasion of the islands so that they could be used as bases for the strategic bombing of Japan by Boeing B-29 Superfortress long-range heavy bomber force that was being created. The seizure of the Mariana islands group would also create a number of other options for the Allies, since bases in these islands would lie within operational range of the Palau, Philippine and Bonin island groups, and also of Formosa. An operation to take the Mariana islands group was now allocated priority over the seizure of the Palau islands group. However, the target date of 15 June 1944 was not set until 12 March 1944 as an objective for Admiral Chester W. Nimitz’s Pacific Ocean Areas command, and on 28 March Nimitz assigned responsibility for the resulting 'Forager' operation to Admiral Raymond A. Spruance’s 5^th Fleet.

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By that time Vice Admiral Richmond K. Turner, the commander of the 5^th Amphibious Force within the 5^th Fleet, had already started the requisite planning. About a week before the 5^th Fleet received its formal directive from Nimitz, Turner had concluded that Saipan should be the island to be invaded first as it possessed the best airfields, and its capture would also sever the Japanese air and maritime links with the islands it still held farther to the south. Turner the 5^th Amphibious Force into two parts as the Northern Attack Force (Task Force 52) under his personal command for the landings on Saipan and Tinian, and Rear Admiral Richard L. Conolly’s Southern Attack Force (TF53) for the landing on Guam.

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The assault elements were to be parts of Lieutenant General Holland M. Smith’s V Amphibious Corps or TF56 (in the form of the Northern Troops and Landing Force’s 2^nd and 4^th Marine Divisions) for the Northern Attack Force, and Major General Roy S. Geiger’s III Amphibious Corps or TF56.2 (in the form of the Southern Troops and Landing Force’s 3^rd Marine Division and 1^st Provisional Marine Brigade) for the Southern Attack Force. The floating reserve under the command of Rear Admiral William H. P. Blandy, was to be Major General Ralph C. Smith’s 27^th Division, and the invasion would be covered by elements of Vice Admiral Marc A. Mitscher’s Fast Carrier Task Force (TF58) and land-based aircraft of Vice Admiral Hoover’s Central Pacific Forward Area or TF57 command. The submarines of Vice Admiral Charles A. Lockwood’s Submarine Force, Pacific Fleet, were to scout well to the west, and logistical support was the responsibility of Vice Admiral William L. Calhoun’s strategically and operationally vital fleet train of the Service Force, Pacific Fleet.

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The defence of the Mariana islands group was the responsibility of Vice Admiral Chuichi Hara’s 4^th Fleet, which was headquartered at Truk atoll in the Caroline islands group. The 5^th Base Force had been established on Saipan before the outbreak of World War II, along with the 5^th Communications Unit and 5^th Defence Force, and these were directly responsible for the defence of the Mariana islands group. Just before the war, moreover, the 5^th Base Force had also been tasked with planning the seizure of the US island of Guam. The 5^th Base Force was redesignated as the 5^th Special Base Force in April 1942, when it had only 1,500 personnel.

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Only the three largest islands (Saipan, Tinian and Guam, all lying toward the southern end of the chain) played major roles in the war, and just minor Japanese installations were located on Rota and Pagan islands.

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As the the commanders of the various US forces were planning their operation against the Mariana islands group, Lieutenant General Hideyoshi Obata’s 31^st Army on Truk was made responsible for the Japanese army elements in the Mariana islands, and this headquarters later moved to the Palau islands group. While the 4^th Fleet had overall responsibility for the area, it had also been decided that once any island was attacked, the senior army officer on that island would assume command of all available forces.

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A major reinforcement of the Mariana islands group was undertaken between February and May 1944, and the islands' comparatively large sizes made it possible for the Japanese to adopt new defensive tactics as, unlike the atolls of the central Pacific, the islands were large enough to open the possibility of defence in depth, though advantage of this fact was not always exploited to the full, and also some degree of manoeuvre. The islands were also small enough for all or most of the main landing sites to be defended, which was a situation which did not prevail on the larger islands of the South Pacific and South-West Pacific areas. Those islands too had limited the defenders' ability to manoeuvre because of their size, dense vegetation and rugged terrain.

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By this time the Allies had decided to embark on a pair of parallel campaigns to break through in the south-west and to penetrate the Pacific defence line in the centre.

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General Douglas MacArthur’s South-West Pacific Area command therefore advanced through New Guinea ('Straightline', 'Horlicks', 'Cyclone' and 'Globetrotter') and onto Morotai island ('Tradewind') in the Halmahera islands of the Japanese-occupied Netherlands East Indies as it advanced toward the Philippine islands, which were to be retaken in a number of 'King', 'Mike' and 'Victor' operations.

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Nimitz’s Central Pacific Area command was to take the Mariana islands group in 'Forager'. As noted above, the selection of the Mariana islands group as a priority target had been influenced by the introduction of the B-29 Superfortress heavy bomber, for in US hands the Mariana islands group would provide a strategic air bombardment springboard within B-29 range of Tokyo and other strategic targets in the Japanese home islands.

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The Japanese were expecting an attack somewhere on their defensive perimeter, but thought that a US onslaught on the Caroline islands group, farther to the south, was more probable, and this was indeed launched later as 'Stalemate II'.

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To reinforce and supply their garrisons the Japanese needed both naval and air superiority, so 'A' was prepared as a major aircraft carrier offensive for implementation as and when needed by June 1944.

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The Mariana islands group lies some 1,000 miles (1610 km) to the north-west of the Marshall islands group, and in World War II were ideally located as a strategic stepping stone for an offensive by the Pacific Ocean Areas toward the Japanese home islands. As noted above, it was also fully appreciated that these substantial islands would provide adequate bases for the fleet of B-29 bombers to be deployed by the US 20^th AAF controlled initially from the continental USA by General Henry H. Arnold. There were already relatively small Japanese airfields on the islands, but the US planners were convinced that three huge air base complexes could swiftly be built to provide a platform from which the US bombers could destroy Japan’s cities and war-making industries to an extent which might possibly remove the need for US forces to invade the Japanese home islands, which all agreed would be an exceptionally costly undertaking. For these reasons, therefore, the successful implementation of 'Forager' was wholly vital to US interests.

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Initially the scheme did not secure the approval of all US commanders in the Pacific theatre: MacArthur’s South-West Pacific Area command desired reinforcements for its proposed recapture of the Philippine islands group, and Nimitz’s Pacific Ocean Areas command was concerned about the threats posed to the US invasion force by lack of a good harbour in the islands and by Japanese land-based air attack in an operational area beyond the reach of any US land-based aircraft.

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In January 1944, MacArthur and Nimitz had met at Pearl Harbor to agree a joint alternative plan for the B-29 bombers to be diverted to MacArthur’s 5^th AAF commanded by Lieutenant General George C. Kenney, and for the naval effort in the central Pacific to be directed at the Caroline and Palau island groups rather than at the Mariana islands group. Nimitz agreed with MacArthur that their forces should then combine for the invasion of the Philippine islands group, starting with Mindanao island.

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Reports of the Pearl Harbor conference angered the US Joint Chiefs-of-Staff in Washington, and as a result Admiral Ernest J. King, the chief of naval operations, rebuked Nimitz in a letter which reminded Nimitz that in the 'Granite' overall strategic plan the Joint Chiefs-of-Staff had already directed the Pacific Ocean Areas command to work toward an invasion of the Mariana islands group as any single drive to the west across the Pacific farther to the south, as envisaged by Nimitz with MacArthur’s support, would leave the US lines of communication open to a Japanese counterstroke from its northern flank.

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An often underestimated but nonetheless essential background feature of 'Forager', and indeed other parts of the central Pacific offensive, was Calhoun’s huge, complex and increasingly efficient system of mobile supply and support 'bases' to keep the combat forces operational and fed with all the requirements of modern war. These floating bases (repair ships, supply ships, tenders, tugs, floating docks and salvage vessels etc) were designed to anchor in the lagoon of a large atoll in an area safe from Japanese submarine and land-based air attack. The floating base moved with the fleet it supported, and in the period from 1943 to 1945, this base thus shifted from Majuro atoll in the southern part of Marshall islands group, to Eniwetok in the northern part of the same group, and then to Ulithi atoll in the Palau islands group.

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Throughout this preparatory phase, carriers struck the Mariana islands group on many occasions. The first of these attacks took place on 23 February, and was the US forces' first good look at the Mariana islands group in more than two years: the raiding aircraft brought back a wealth of photographic intelligence. The raid also destroyed 168 Japanese aircraft and sank 45,000 tons of shipping. Land-base aircraft of the 5^th, 7^th and 13^th Army Air Forces, mostly heavy bombers conducting night raids, bombarded Japanese bases in the Caroline islands group throughout March to ensure there would be no Japanese interference with 'Forager' from the south. Starting on 18 April, Consolidated PB4Y-1P long-range photo-reconnaissance aircraft, based on the B-24 Liberator heavy bomber, of three Guadalcanal-based squadrons started to stage through Eniwetok to map the Mariana islands group, and these were joined by the aircraft of another squadron based on Eniwetok itself.

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The whole of the 'Forager' campaign presented formidable operational problems, and powerful forces were assembled for the task, which on 28 March was entrusted to Spruance’s 5^th Fleet. Spruance assembled his command in the lagoon of Eniwetok atoll, and all was prepared for Turner’s TF51 (Joint Expeditionary Force) to assault the islands with the 127,000 men of Smith’s V Amphibious Corps after the defences had been softened by the final attacks delivered by the carrierborne aircraft of Mitscher’s TF58, which destroyed some 200 Japanese aircraft and sank 12 cargo ships.

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For this wide-ranging sweep, which lasted from 11 to 17 June, TF58 1 operated as four carrier task groups. On 11 June all four carrier task groups had carried their first fighter attack on all of the major islands of the Mariana group, and destroyed 36 Japanese aircraft. TG58.4 attacked a convoy which had just departed Saipan and sank the torpedo boat Ootori, three submarine chasers and 10 merchant ships (30,000 tons). On 12 and 13 June TG58.2, TG58.3 and TG58.4 attacked Saipan and Tinian while TG58.1 attacked Guam. However, on 14 and 15 June there were only individual sorties over the Mariana islands group as the ships of TG58.2 and TG58.3 were replenished. Off to the north, TG58.1 and TG58.4 attacked Iwo Jima, Chichi Jima and Haha Jima in the afternoon of 15 June and on 16 June. On the way to the rendezvous with the rest of TF58, TG58.4 attacked the island of Pagan again on 17 June.

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The US carrierborne air attacks came as a rude shock to the Japanese, who had not imagined that the Americans could exploit their 'Cataract' ('Flintlock' and 'Catchpole') and 'Hailstone' successes in the Marshall islands group and against Truk so speedily, and who in any case believed that the Palau islands group would be the next US target.

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The Americans had also carried out the major 'Wedlock' deception campaign to suggest their next major assault would come in the Kurile islands group to the north of Japan’s four major home islands.

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For 'Forager', the US plan involved two main groups of amphibious forces, namely TF52 and TF53. The former was the Northern Attack Force under Turner’s command, and would deliver onto Saipan (and then Tinian) the 66,800 men of the Northern Troops and Landing Force under Smith’s personal command and comprising Major General Thomas E. Watson’s 2^nd Marine Division (2^nd, 6^th and 8^th Marines plus the 10^th Marine Artillery) and Major General Harry Schmidt’s 4^th Marine Division (23^rd, 24^th and 25^th Marines plus the 14^th Marine Artillery), with Major General Ralph C. Smith’s 27^th Division (105^th, 106^th and 165^th Infantry) as floating reserve: Ralph Smith was replaced by Major General George W. Griner during the Saipan fighting at the express order of Holland Smith.

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The combined artillery for the 'Forager' operation was the responsibility of the US Army’s XXIV Corps as the III Amphibious Corps' own artillery had been diverted to support the XXIV Corps for the cancelled operation against Yap island on the north-western edge of the Caroline islands group.

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Saipan is the second largest island in the Mariana group, and in 1944 possessed a fairly extensive road network and narrow-gauge railway system. The island has an area of 44.55 sq miles (115.4 km²), and is about 13 miles (21 km) long and 5.6 miles (9 km) wide. The centre of the island is rugged, and rises to a height of 1,560 ft (475 m) at Mt Tapochau. The southern end of the island is a plateau at a height of about 200 to 300 ft (60 to 90 m), and some 70% of the island was devoted to the farming of sugar cane, the rest being scrub or grassland.

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There was a large Japanese civilian population, numbering about 20,000, plus 4,000 Chamorros and 100 Kanakas. The town of Garapan was the Japanese administrative centre of the Mariana islands group and had a population of some 10,000, and just to the north of this the Japanese had developed Tanapag harbour, a reef-protected anchorage. Tanapag had a depth of 20 to 50 ft (6 to 15 m) and the Japanese had dredged a clear channel to the anchorage and built four large piers up to 700 ft (210 m) long.

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Located toward the island’s southern end, Aslito airfield dated from the 1930s and was the most important Japanese air base between Japan and Truk. A 4,380-ft (1335-m) airstrip was under construction at Marpi Point on the northern end of the island, and an airstrip 3,875 ft (1180 m) long and 150 ft (45 m) wide had been built at Charan Kanoa, but this was was aligned crossways to the prevailing winds, making it suitable for use only by very light aircraft.

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The island’s coastal defences were still incomplete in June 1944, but at that time included eight 6-in (152-mm) guns, nine 140-mm (3.54-in) guns, eight 120-mm (4.72-in) dual-purpose guns, four 200-mm (7.87-in) mortars, and about 25 concrete blockhouses and pillboxes. Not all the guns had yet been sited, and significant quantities of boat mines, barbed wire and other defence material had arrived but not yet been put to use. Even larger quantities of construction materials had been sunk by the US submarine blockade.

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US intelligence estimated that the defence force available on Saipan to Vice Admiral Chuichi Nagumo’s Central Pacific Area Fleet and Obata’s 31^st Army was some 20,000 men, though the actuality was 25,470 soldiers including those of Lieutenant General Yoshitsugu Saito’s 43^rd Division (118^th, 135^th and 136^th Regiments) which doubled as the Northern Marianas Army Group responsible for the defence of Saipan and Tinian, Colonel Yoshiro Oka’s 47^th Independent Mixed Brigade (ex-1^st Expeditionary Unit) and a miscellany of smaller units including two infantry battalions shipped in from other islands and the 9^th Tank Regiment, as well as 6,160 sailors and naval troops including the regimental-size 41^st and 55^th Base Forces, and the battalion-size Yokosuka 1^st Special Naval Landing Force. There were also a number of artillery, anti-aircraft, tank, engineer and service units.

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Many of Saito’s formations and units had reached Saipan without their full complements of equipment as a result of the success of US attacks on the reinforcement convoys. In particular, the 118^th Regiment had lost 858 men and almost all its equipment when its convoy lost five of eight ships to a US submarine wolfpack on 4/6 June 1945.

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This first element of the 'Forager' campaign started with the aerial bombing and naval gunfire bombardment of Saipan from 13 June. The naval bombardment force included 15 battleships, and in the course of the bombardment 165,000 shells were fired. As this preliminary programme was pressed forward, the assault force closed on Saipan.

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Within TF52 were the heavy surface warships of Rear Admiral Jesse B. Oldendorf’s TG52.17 2 and Rear Admiral Walden L. Ainsworth’s TG52.10 3; and for the provision of air support Rear Admiral Gerald F. Bogan’s TG52.14 4 and Rear Admiral Harold B. Sallada’s TG52.11 5.

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On 14 June the battleship California and destroyer Braine were damaged by fire from shore batteries off Saipan and Tinian respectively, while the battleship Tennessee, cruisers Indianapolis and Birmingham, and destroyers Remey and Wadleigh suffered lesser damage from near-misses. On 15 June the battleship Tennessee again and the cruiser St Louis were damaged, as were LCI(G)-451 and LCI(G)-726. On 22 June the battleship Maryland was struck and damaged by an air-launched torpedo, the tank landing ship LST-119 by shore gunfire and the transport Prince Georges by a bomb near-miss.

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As the vanguard of the invasion fleet, the fast carriers began air attacks on the Mariana islands group during the afternoon of 11 June. About 36 Japanese aircraft were destroyed, and a torpedo attack on the night of 11/12 June by Mitsubishi G4M 'Betty' bombers was repulsed with one 'Betty' shot down. By the fall of night on 13 June, Japanese air power on Saipan and its neighbouring islands had been all but eliminated. Two night attacks on 15 June from Guam were largely broken up, the Japanese losing at least seven aircraft and inflicting no damage on the US ships.

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Lee’s battleship force began its preliminary gunfire bombardment on 13 June. The first day was largely unsuccessful, as the battleships' gunners had trained largely for fleet action rather than shore bombardment, and were firing high-velocity guns with too flat a trajectory. It did not help that the ships had to fire from a range of more than 10,000 yards (9145 m) as the fast battleships were considered too valuable to risk in shallow water that had not yet been swept for mines. Some 2,432 16-in (406-mm) and 12,544 5-in (127-mm) shells were expended to little effect.

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On the following day, the fast battleships were relieved by the fire support groups, whose older battleships had trained extensively for shore bombardment and carefully followed a detailed fire plan. The ranges were also considerably shorter as the old battleships were considered more expendable, and some of the old battleships fired from as close as 3,500 yards (3200 m) to the coast. In all, the preliminary gunfire bombardment dropped a total of almost 2,430 tons of shells onto the island.

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Two underwater demolition teams began operating off the landing beaches under cover of the bombardment of 14 June bombardment. These teams reported on water depth and the location of reefs in the landing areas, pinpointing paths through the reefs for landing craft. To their surprise, no boat obstacles or mines were encountered. The UDTs returned that evening to blast out boat passages and ramps with high explosive charges.

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On 15 June Captain Knowles’s TG52.3 transport group of 13 attack transports, five cargo ships and one dock landing ship landed the 2^nd Marine Division, and Captain Loomis’s TG52.4 transport group of 13 attack transports, three attack cargo ships and three dock landing ship landed the 4^th Marine Division.

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Both divisions were veteran formations, the 2^nd Marine Division having fought on Guadalcanal and Tarawa, and the 4^th Marine Division at Roi-Namur.

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The two landing groups were escorted by the destroyers Newcomb, Bennion, Heywood L. Edwards, Bryant, Prichett, Philip, Cony, Mugford, Selfridge, Ralph Talbot, Patterson, Bagley, Phelps, Shaw and Renshaw. Further assault waves arrived on landing ships and boats. In all, TF52 consisted of 551 ships, and in total some 67,451 men were disembarked.

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Within this overall effort, and supported by a feint landing to the north of Garapan, the landings began at 07.00 on 15 June after a final two-hour gunfire bombardment and a 30-minute air attack. A force of more than 300 LVTs and numerous landing craft delivered 8,000 US Marines from 08.44 to about 09.00 onto eight beaches between points to the south of Garapan and to the south of Charan Kanoa on the southern end of Saipan’s western coast, with the 2^nd and 4^th Marine Divisions on the left and right of the beach-head respectively.

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The LVTs in particular suffered heavy casualties from Japanese artillery, which had been carefully emplaced on reverse slopes overlooking the beach and had registered points throughout the landing area, which included flags placed in the bay to indicate the range. Pillboxes on Afetna Point, in the centre of the landing area, and Agingan Point, at the southern end of the landing beaches, had survived the preliminary gunfire and air bombardments, and now laid down fire so heavy that some US landing craft shied away and landed their troops elsewhere, splitting the beach-head. The landing force was also subjected to enfilading fire from Afetna Point and Agingan Point.

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All four of the US battalion commanders had been wounded within a few hours, together with more than 2,000 of their men, and the marines managed to reach only about half of their first-day objectives, but by the fall of night the marines had established a beach-head about 6 miles (10 km) wide and 1,100 yards (1000 m) deep, and landed some 20,000 men as well as tanks and artillery.

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The failure to link the two beach-heads left the marines facing the possibility of defeat in detail, but the Japanese missed their opportunity to do this by attacking both of the beach-heads at the same time. A concentration of about 1,000 Japanese troops to the north of the northern beach-head was hit by 5-in (127-mm) shells from the battleship California at 17.12, but this did not prevent further probing attacks. The main Japanese counterattack in the north came at 03.00 and lasted almost three hours. Five marine tanks finally stopped the Japanese at sunrise with the aid of cruiser and destroyer gunfire. About 700 Japanese were killed. The second counterattack, from the south, was preceded by artillery preparation and hit the 25^th Regiment, but was shattered by 105-mm (4.13-in) artillery fire. Marines later claimed that the Japanese mingled women and children with their ranks to create the impression that this was group of civilians attempting to surrender. The third counterattack came through the gap between the two beach-heads at about 05.30 and briefly took the Charan Kanoa pier. The Japanese were driven back with heavy loss, but only after they had badly damaged the valuable pier.

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By 04.00 on 16 June Spruance had been informed that the Japanese fleet was approaching the Mariana islands group in 'A' and, after consulting Turner and Smith, decided on the immediate commitment of the to commit the reserve, the 27^th Division. This began come ashore during the evening in the area to the immediate south of the 4^th Marine Division, and advanced on Aslito airfield.

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The Japanese made another counterattack at 03.30 on the next morning with a force of 500 men supported by 44 tanks. Ships offshore fired star shells that brilliantly illuminated the Japanese attack, and the marines engaged the Japanese tanks with bazooka rocket-launchers, 37-mm anti-tank guns, and grenades. These were joined by 75-mm (2.95-in) guns on half-tracked chassis at dawn. The Japanese tanks were annihilated, the last survivor being destroyed by 5-in (127-mm) naval gunfire as it attempted to escape into the hills.

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Less than one hour later, the marines went over to the offensive, and by the end of 17 June had doubled the size of their beach-head. By 14.00 elements of the 165^th Infantry had reached Aslito airfield with the support of the marines' artillery whose fire was spotted by Stinson OY-1 Sentinel air observation post light aircraft launched from escort carriers, while deeper fire support continued to be supplied by the heavier guns of the warships. Some 85% of all observed artillery fire on Saipan was directed by light observation aircraft.

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During the evening of this day the Japanese launched air raids from Truk and Yap, damaging two landing ships and putting a bomb through the after elevator of the escort carrier Fanshaw Bay, which was compelled to retire to Eniwetok for repairs. The Japanese lost three aircraft, while the Americans lost six aircraft from White Plains in a landing accident.

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By 18 June Saito was forced to admit that he had no hope of driving the Americans back into the sea, and ordered most of his units to pull back to a line passing through Mt Tapotchau, allowing the marines to clear most of the southern part of the island and begin repairing Aslito field, which the marines renamed Isely Field. Here the first US warplanes, Republic P-47 Thunderbolt heavy fighters of the 19^th Fighter Squadron and Northrop P-61 Black Widow night-fighters and intruders of the 73^rd Fihter Squadron, landed on 22 June.

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Although the invasion had surprised the Japanese, who had been expecting an attack farther to the south, Admiral Soemu Toyoda, commander-in-chief of the Japanese navy, nonetheless saw an opportunity to use the 'A' force to attack the US Navy forces around Saipan and on 15 June gave the order to attack. But the resulting Battle of the Philippine Sea was a disaster for the Japanese navy, which lost three aircraft carriers and hundreds of aircraft, effectively removing any opportunity for the reinforcement and resupply of the Japanese garrisons of the Mariana islands group.

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During the Battle of the Philippine Sea, which was fought in the waters to the west of the Mariana islands group, most of the US amphibious fleet was withdrawn to safer waters to the east of the islands. A few transports were allowed to return and continue unloading supplies on 19 June, and the remainder of the force returned on 21 June, after the Americans had won the fleet engagement.

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Without resupply, the defenders of Saipan faced a hopeless situation, but determined to fight to the last man. Saito organised his troops into a line anchored on Mt Tapotchau in the defensible mountainous terrain of central Saipan. The nicknames given by the Americans to the features of the battle, such as 'Hell’s Pocket', 'Purple Heart Ridge' and 'Death Valley', indicate the severity of the fighting. The Japanese used the many caves in the volcanic landscape to delay the attackers, by hiding during the day and making sorties at night, but the Americans gradually developed tactics for clearing the caves by using flamethrower teams supported by artillery and machine guns.

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It took another two weeks of slow, grinding battle for the Americans to clear the remaining Japanese troops from Saipan. The Japanese made occasional small air raids: it was one of these which secured a torpedo hit on Maryland during 22 June. In the period 21/26 June the Americans attempted to batter their way past Mt Tapotchau, and the 27^th Division, now in the centre between the 2^nd Marine Division on the west coast and 4^th Marine Division on the east coast, began to lose the momentum of the advance as it encountered the formidable terrain and well-sited man-made defences around 'Purple Heart Ridge' and 'Death Valley', began to lag. Holland Smith had been disappointed with the performance of 27^th Division at Makin, and his opinion of the division was worsened still further as it failed to keep up with the marine divisions on its flanks. Holland Smith relieved Ralph Smith from command of the 27 Division on the evening of 24 June, replacing him with Major General George W. Griner in very controversial circumstances.

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On 27 June, some 500 Japanese troops, who had been contained in the southern tip of the island, managed to slip past a battalion of the 27^th Division, raided Isely Field and then headed to the north in search of Saito’s command post. The Japanese then encountered the 14^th and 25^th Marines and were annihilated, at the cost of 33 marine casualties. On the same day, men of the 2^nd Marine Division reached the summit of Mt Tapotchau.

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On 4 July the 2^nd Marine Division took Garapan. Having pulled back to Marpi Point on the extreme northern tip of the island by 6 July, the Japanese had no further place to which they could retreat. Saito ordered his remaining able-bodied troops, totalling some 3,000 men, forward in a suicide charge, and then killed himself; so took did Nagumo. This was the largest suicide charge of the war, and took the 27^th Division by surprise despite the fact that Smith had warned Griner that an attack of this type was probable. The banzai charge ended organised Japanese resistance on the island, which was declared secure on 9 July, though small groups of Japanese soldiers were hunted down until 10 August.

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By the time of the island’s capture on 9 July, Japanese losses were at least 23,811 dead with 1,780 men taken prisoner, and US losses were 3,426 dead or missing and 13,099 wounded or, according to other sources, 3,126 US dead (out of total casualties of 14,111) and about 30,000 Japanese including Nagumo and Saito, the latter commanding in the absence of Obata who was on an inspection tour in the Palau islands group at the time of the US invasion. Within the Japanese total were some 8,000 civilian dead, most of them ethnic Japanese who committed suicide between 9 and 12 July, some jumping from 'Suicide Cliff' and 'Banzai Cliff' despite futile efforts by US troops to dissuade them.

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The loss of Saipan was a great shock in Japan, and precipitated the fall of General Hideki Tojo’s cabinet.

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Saipan then became an important US base for further operations in the Mariana islands group, and then for the invasion of the Philippine islands group in October 1944. US bombers based on Saipan started to attack targets in the Philippine islands group, the Ryukyu islands group and the Japanese home islands. The island was rapidly developed into a permanent base, which included an oil tank farm with a capacity of 100,000 barrels (13,000 tons). The captured Japanese facilities were repaired and extended and the airstrip at Marpi Point was completed.

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On 25 December, Isely Field was raided by five Yokosuka P1Y 'Frances' bombers of the 501^st Kokutai and several Mitsubishi Ki-67 'Peggy' bombers of the 7^th Sentai. The Japanese aircraft destroyed four B-29 Superfortress bombers and damaged another eleven at the cost of two P1Y warplanes.

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The US forces' next step was the descent on Guam, the completion of whose capture was the last stage of 'Forager', so the final stage of 'Forager' proper was therefore the assault on Tinian, just to the south of Saipan, which was one of the reasons why the Saipan assault force was also used for the Tinian operation.

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Tinian is the third largest island of the Mariana group after Guam and Saipan, with a length of about 10.5 miles (17 km) and an area of 39 sq miles (101 km²). Unlike most of the group’s other islands, Tinian is largely flat with the exception of some low hills in the south-western area and near the north-west coast. The latter includes the highest point on the island, the 561-ft (171-m) Mt Lasso, which was the location of the Japanese command post.

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The civilian population in 1941 was about 18,000 persons, almost all of them Japanese and Okinawans. About 90% of the land area was under cultivation, and sugar production was sufficient to support two refineries connected to the cane fields by a narrow-gauge railroad. In addition, the island was covered with a grid of narrow earth roads between the sugar cane fields.

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The island lacked a good port, but its flat terrain made it ideal for the location of airfields, and the Japanese had completed one airfield in the north at Ushi Point with two 4,700-ft (1430-m) runways and had three other airfields under construction by June 1944.

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Detailed planning for the US assault on Tinian began only in June 1944, when tactical reconnaissance was carried out even as Saipan was being conquered. The plan called for a shore-to-shore operation using the same forces as had been used in the invasion of Saipan. Preliminary bombardment by warships and aircraft which were not required for the Saipan battle began on 11 June, and from 20 June this was supplemented by the heavy guns of the XXIV Corps' artillery sited on the south coast of Saipan to fire across the Saipan Channel. The preliminary bombardment included the first field trials of napalm bombs.

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The best landing beaches, on the west coast in the area of Tinian town, were defended by the largest concentration of Japanese troops and were therefore soon ruled out of contention. The alternatives were the Yellow beaches the north-east coast in Asiga Bay and the White beaches on the north-west coast to the south-west of Ushi Point. A reconnaissance by frogmen on 10/11 July established that the Yellow beaches were heavily defended, with numerous mines and fortifications, and also a difficult reef. The two White beaches were just 60 yards (55 m) and 160 yards (150 m) wide, but they were tactically attractive as they were neither mines nor a barrier reef. After initially voicing strong opposition, Turner decided to risk a landing over the two White beaches. Improvised ramps were designed for placement by LVTs on the low cliffs at the ends of each beach to allow other LVTs to come ashore here, thereby increasing the effective width of the assault beaches. Every effort was made to ensure that the assault units moved off the narrow beaches as rapidly as possible so that they could reach areas over which they could immediately fan out. The plan called for an assault by 14 waves each of 24 LVTs.

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Much consideration was given in the planning process to the possibility of foul weather following the landings. The typhoon season had begun, and a major typhoon immediately following the landing would have played havoc with the logistics for the White build up. In fact the first period of bad weather did not arrive until five days after the landing.

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Holland Smith had been elevated to Commanding General, Fleet Marine Force, Pacific Fleet, and command of the V Amphibious Corps had passed to Schmidt, Major General Clifton B. Cates thereupon assuming command of the 4^th Marine Division.

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Under the control of the Northern Marianas Army Group on Saipan until 7 July, when it was transferred to the supervision of the Southern Marianas Army Group on Guam, the Japanese garrison on Tinian amounted to some 4,700 men of Colonel Kaishi Ogata’s 50^th Regiment (reinforced to four battalions by the addition of one battalion of the 135^th Regiment from Saipan) of Lieutenant General Takeshi Takashina’s 29^th Division, whose other two regiments held Guam, 350 miscellaneous army personnel, one company of light tanks, and 4,110 marines and sailors of Captain Goichi Oya’s 56^th Guard Force as well as construction and service personnel. Overall direction of the defence of Tinian was exercised nominally by Vice Admiral Kakuji Kakuda, commander of the 1^st Air Fleet, but this officer in fact played no part in the control of the land battle, and the actual conduct of the defence was severely affected by bitter inter-service rivalry between Ogata and Oya.

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It is worth noting that the fighting on Tinian was characterised by the first use of napalm in battle. Of the 120 jettisonable tanks dropped by US warplanes during the operation, 25 contained the napalm mixture and the remainder an oil/petrol mix. Of the entire number only 14 failed, and eight of these were then ignited by strafing runs. Carried by Republic P-47 Thunderbolt fighter-bombers, the 'fire bombs' were used mainly to burn away foliage concealing Japanese positions.

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Rear Admiral Harry W. Hill’s TF52 moved and landed the relevant formations of Schmidt’s V Amphibious Corps, whose 15,614 men were delivered by elements of TG52.4, tank landing ships and smaller landing craft. Fire support was provided by most of Oldendorf’s TG52.17 and Ainsworth’s TG52.10, and air support by Bogan’s TG52.14 and Sallada’s TG52.11. In the shelling, the battleship Colorado, destroyer Norman Scott and tank landing ship LST-481 were hit and damaged by the fire of the Japanese coastal artillery. Though the island was secured largely by men of the US Marine Corps, it is worth noting that US Army aircraft, artillerymen, amphibian vehicles and engineers helped invaluably toward the success of the Tinian operation.

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Just 3.5 miles (5.6 km) to the south of Saipan across the Saipan Channel, Tinian saw the landings of the 4^th Marine Division over the White beaches on the island’s north-west coast as the 2^nd Marine Division feinted toward Tinian town in the south-west of the island before following the 4^th Marine Division. The demonstration went as far as embarking troops in landing craft and having them close to within 1 mile (1.6 km) of the shore before turning back, which led the over-optimistic Japanese to believe that they had repelled the landing.

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The operation was supported, as usual, by a naval bombardment and, in this instance, artillery firing across the Saipan Channel from the southern portion of Saipan. The feint toward Tinian town was successful in diverting defenders from the actual landing site on the north of the island. Here the Japanese were caught completely off guard by the real landings, which began at 08.00 and saw the landing of three battalions in just 20 minutes. The only serious resistance was provided by a pair of pillboxes, which were bypassed for later destruction. By 10.30 bulldozers had come ashore, and within one hour the first tanks were able to land. The marines' artillery was ashore by 14.15.

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Three Japanese counterattacks were beaten back during the night of 24/25 July. The first, by about 600 poorly trained naval infantry against the northern end of the marines' beach-head, began just after 24.00 and was annihilated, the marines counting 476 bodies in front of their lines during the morning of the following day. The second counterattack, against the centre of the beach-head perimeter, was launched at 02.30 and hit the junction of two regiments, achieving a significant penetration and causing brief concern before it, too, was destroyed with the loss of about 500 men. The last counterattack hit the southern end of the marine perimeter and was supported by five light tanks, which were quickly destroyed by marine bazooka fire and artillery. The surviving Japanese infantry committed suicide at daybreak in spectacular fashion with the magnetic anti-tank mines they were carrying.

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The Japanese adopted the same stubborn defensive tactics as on Saipan, retreating during the day and attacking at night. The gentler terrain of Tinian allowed the attackers more effective use of tanks and artillery than in the mountains of Saipan, however, and the island was secured in only nine days of fighting as the attacking forces drove south with the 2^nd and 4^th Marine Divisions on the left and right respectively. On 26 July the marines reached Ushi Point, and on 31 July the surviving Japanese launched a suicide charge. The island was declared secure on 1 August after what Smith declared to have been 'the perfect amphibious operation in the Pacific War'. Even so, several hundreds of Japanese troops held out in the jungles for months, and the garrison on Aguijan island off the south-western cape of Tinian, commanded by Lieutenant Kinichi Yamada, held out until the end of the war, surrendering only on 4 September 1945.

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The US casualties totalled 389 men killed and 1,816 wounded, while those of the Japanese were 6,050 known killed and 236 taken prisoner; the precise fate of the other Japanese defenders remains unknown, but these were in all probability buried in demolished caves and bunkers.

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After the fighting had died down, Tinian was developed as an important base for further US operations in the Pacific War. Camps were built for 50,000 troops, and 15,000 Seabees (one full brigade of six battalions) turned the island into the busiest airfield complex of the war, levelling the high ground and completing six main runways, each 8,500 ft(2590 m) long, to support B-29 operations. The work involved the movement of 11 billion cubic yards of earth and coral and using vast amounts of construction equipment and materials. From these runways B-29 bombers lifted off to attack targets in the Philippine islands group, the Ryukyu islands group and the Japanese home islands, the bombers directed at targets in the last generally taking off with incendiary bombs (oil, napalm and white phosphorus) and, in the case of the 'Silverplate' and 'Centerboard' attacks against Hiroshima and Nagasaki on 6 and 9 August 1945, atomic bombs.

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The last stage of 'Forager' to be completed, though it began before the descent on Tinian, was the recapture of Guam between 27 July and 10 August.

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Guam is the largest island of the Marianas group, with a length of 32 miles (51 km) and an area of 210 sq miles (544 km²), representing an area greater than that of all the other of the group’s islands combined. The island has a damp climate, with daily rains during the wet season from July to December, and the temperature ranges between 20.5° and 33° C (69° and 91° F). The southern half of the island is mountainous along its western side to the south of Agana, the main town, with a maximum height of 1,334 ft (407 m) at Mt Lamlam. The northern part of the island is a relatively flat plateau at a height of between 400 and 500 ft (120 and 150 m), and ends in high cliffs close to the shore. The island is more heavily vegetated than the other Mariana islands, with the higher peaks heavily forested and the remainder covered with scrub and grassland. There are numerous streams draining toward the south-east coast, and some of the low ground in the southern half of the island is swamp. The northern plateau is so heavily vegetated that it was actually less developed than the rugged southern half, where most of the cultivated land was found.

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The only portion of Guam’s shore suitable for amphibious landings was the south-west coast, where the offshore reef was less formidable and there were no high cliffs immediately behind the beaches.

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Unlike the rest of the Mariana islands group, which belonged to Japan, Guam was a US territory in 1941, having been purchased from Spain as part of the settlement of the Spanish-American War in 1898. The USA did little to develop Guam as a military base, for despite its strategically important location in the western Pacific, its port at Apra was limited in size and accessibility, and also shallow, and the island was in a very exposed position. Furthermore, the Washington Naval Treaty of 1922 prohibited any US fortification of Guam. However, the Americans did construct Piti Navy Yard on the western end of the harbour and build a causeway to Cabras island, which lies to the north of the harbour. Here the facilities included an 800-yard (730-m) pier at the south-western end of Cabras island, a breakwater, and numerous jetties and fuelling docks.

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Fortification of Guam was considered after the lapse of the treaty, but the island’s isolated and wholly exposed location meant that it was in effect written off by military planners.

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The US population was small, comprising mostly administrative and naval logistic personnel and their families, and there was little economic development apart from the fuelling station. Efforts were made to improve health care and education for the Chamorro native population, which was otherwise left largely undisturbed. The population in 1941 was 23,400, of whom 12,550 lived in Agana, the island’s capital.

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Like Wake island to its north-east, Guam was a staging post between the major US facilities on the US west coast and the Hawaiian islands to the east, and the major US possession of the Philippine islands group to the west. Thus the Japanese had planned the seizure of Wake and Guam islands even before they turned their attention to the desirability of destroying the US Pacific Fleet at Pearl Harbor in 'Ai'. Both islands were to be taken by Vice Admiral Shigeyoshi Inouye’s 4^th Fleet based at Truk, the principal Japanese mid-Pacific naval base in the Caroline islands group.

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From March 1941 Japanese aircraft flew reconnaissance sorties over the island, and the plan for the invasion of the island had been completed by September of the same year. The unit selected for the operation was Major General Tomitaro Horii’s South Seas Detachment, centred on the 144^th Regiment and other units detached from Lieutenant General Takeshi Koga’s 55^th Division. The maximum strength available was 4,886 men. The South Seas Detachment was concentrated in Korea during November 1941 and, after a brief stay in Japan, sailed for Chichi-jima in the Bonin islands group late in the same month. The 370-man 5^th Company of the 2^nd Maizuru Special Naval Landing Force, based on Saipan in the Mariana islands group, was also assigned to the assault.

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The Japanese forces were transported to Guam by nine transport vessels escorted by the minelayer Tsugaru and four destroyers. The four heavy cruisers of Rear Admiral Aritomo Goto’s 6^th Cruiser Division were also available to provide support if needed, and the landing force and naval units were supported by the Imperial Japanese navy’s 18^th Koku Sentai, which was based at Saipan and equipped only with floatplanes.

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The USA believed that it was neither possible nor practical to attempt any real defence of Guam. The island was not seen as being of utility in US efforts to reinforce the Philippine islands group, though it served as a refuelling point for Pan American’s commercial flying boat services and was also a relay points for the Pacific Cable Company’s telegraph cable linking the Philippines and the US west coast. In 1941 the island’s defence rating was so low that the construction of new defences was not authorised and laid it down that the island’s defenders were to destroy all facilities of military value and withdraw should war break out. Despite the island’s low priority, however, some minor steps were taken to improve the island’s defences before war broke out. A contract for minor improvements to the military facilities on Guam was issued in April 1941, and work began during the following month. The Guam Insular Force Guard, which was a locally manned militia force responsible for protecting the naval base, was also slightly expanded in May.

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On 17 October the dependents of US military personnel were evacuated by sea to the USA, and these were followed by more than 1,000 construction workers.

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On 23 October 1941 the US Navy provided Secretary of the Navy Frank Knox with a report on the defences of Guam, and recommended against any reinforcement of an island which presented immense logistical problems of defence at a time when resources were more practically accorded to other areas. The report did also argue in favour of continuing to improve Guam’s harbour and seaplane facilities, however.

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On the outbreak of war on 8 December 1941 (local time), Guam was defended by a small number of US Navy, US Marine Corps and Insular Force Guard personnel. Captain George J. McMillin, the US Navy officer who was also the island’s governor and overall commander, headed the Naval Forces, Guam, which comprised 271 largely unarmed personnel and four nurses. This force was subordinate to the Vice Admiral Thomas C. Hart’s Philippines-based Asiatic Fleet. Guam’s guard ship, the 5,066-ton auxiliary Gold Star, had sailed to the Philippines to collect supplies and was ordered to remain there. The minesweeper Penguin was at the island, however, along with the immobile oil depot ship Robert L. Barnes. The Marine Barracks, Sumay had a strength of 145 men organised into a company armed with rifles and a small number of machine guns. The Insular Force Guard comprised 246 men, most of whom had received little training. In addition to these military units, Guam’s police force (the Guam Insular Patrol) had 80 men armed only with pistols.

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At 04.45 on 8 December, McMillin was informed about the 'Ai' attack on Pearl Harbor. At 08.27, Japanese aircraft from Saipan attacked the Marine Barracks, the Piti Navy Yard, Libugon radio station, the Standard Oil Company’s installation, and the Pan American Hotel, and also sank Penguin, which lost one officer killed and several men wounded. The air raids all over Guam continued to 17.00. At 08.30 on the following day the Japanese air attacks resumed, with no more than nine aircraft attacking at any one time. The same targets were attacked, together with the Government House in Agana and several villages.

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During the evening of the same day, a Japanese invasion force of four heavy cruisers (Aoba, Kinugasa, Kako and Furutaka), four destroyers (including Kikuzuki, Uzuki and Yukuzi), two gunboats of the 7^th Gunboat Division, six submarine chasers of the 59^th and 60^th Submarine-Chaser Divisions, two minesweepers of the 15^th Minesweeper Division and two tenders departed Saipan. The Japanese ships landed 370 men of the 2^nd Maizuru Special Naval Landing Force from Saipan on 10 December at Dungcas Beach, to the north of Agana. They attacked and quickly defeated the Insular Force Guard in Agana before advancing on Piti, and moving toward Sumay and the Marine Barracks. The principal engagement took place on Agana’s Plaza de Espana at 04.45 when a few marines and Insular Force Guardsmen fought with Japanese marines.

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After token resistance, the marines surrendered at 05.45, and McMillin officially surrendered at 06.00. A few skirmishes occurred in other parts of the island before news of the surrender spread and the rest of the island forces laid down their arms. The US patrol boat YP-16 was scuttled, and YP-17 was captured by the Japanese. Meanwhile the South Seas Detachment made separate landings at Tumon Bay in the north, near Merizo on the south-west coast, and at Talafofo Bay on the east coast.

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The US Marine Corps lost five men killed and 13 wounded, the US Navy eight men killed, and the Guam Insular Force Guards four men killed and 22 wounded. One Japanese marine was killed and six wounded. The US military and civilian personnel were held for a month on the island and then shipped to detention in Japan.

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The Imperial Japanese navy then renamed the island Omiya Jima (Great Shrine Island), made Japanese the language of instruction in the local schools, and put most of the adult male population to work on the construction of airfields. There was one airfield on the Orote peninsula, which forms the southern shore of Apra Harbor; a second to the north-east of Agana; and a third still farther to the north-east that was never completed.

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Japanese treatment of the Chamorros deteriorated farther after the 1944 arrival of army personnel to garrison the island, all schools and churches were closed, and the food supply confiscated. Hundreds died in concentration camps and a number of Chamorros were executed for sympathising with the Americans.

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The US forces originally scheduled Guam for invasion on 18 June 1944, but the landings were postponed as a result of the Battle of the Philippine Sea, and because the difficulties encountered in taking Saipan served to convince the US leadership that three formations would be needed to achieve the recapture of Guam: the third division added to the invasion force at this time was Major General Andrew D. Bruce’s 77^th Division, currently based at Oahu in the Hawaiian islands group as the Pacific Ocean Areas' general reserve. The delay allowed the Japanese to construct elaborate beach defences, but also provided the US forces to undertake both a thorough and systematic reconnaissance and a major ship and aircraft bombardment of the Japanese defences.

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The latter began on 8 July under the command of Conolly, who also led TF53, the invasion force. Aerial reconnaissance sorties were flown every morning to allow a combined team of six navy and marine officers to evaluate the results of the previous day’s effort. The ammunition expenditure in the shore bombardment programme was 6,258 16-in (406-mm) and 14-in (356-mm) rounds from the battleships, 3,862 8-in (203-mm) rounds from the heavy cruisers, 2,430 6-in (152-mm) rounds from the light cruisers, and 16,214 5-in (127-mm) rounds largely from the destroyers in what was the most prolonged and systematic bombardment of the war. A Japanese officer who escaped into the bush and surrendered after the Japanese surrender told his interrogators that the bombardment destroyed all the coastal defences in the open and about half of all emplacements under cover, as well as proving a severe test of Japanese morale.

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Beach reconnaissance was carried out by Underwater Demolition Team 3 on 14 and 15 July 1944, and this team’s frogmen and swimmers discovered the great extent of the Japanese beach defences. These comprised primarily of coral and concrete landing craft obstacles, sometimes connected by steel cables. Frogmen began the demolition of obstacles on 17 July in an effort which had destroyed 300 coral cribs and 640 wire and coral cubes by the time of the landings.

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The recapture of Guam was undertaken by Conolly’s TF53, whose 54,690-man Southern Troops and Landing Forces comprised Major General Roy S. Geiger’s III Amphibious Corps with Major General Allen H. Turnage’s 3^rd Marine Division (3^rd, 9^th and 21^st Marines plus the 12^th Marine Artillery) and Brigadier General Lemuel C. Shepherd’s 1^st Provisional Marine Brigade (4^th and 22^nd Marines) as the assault force with Bruce’s 77^th Division (305^th, 306^th and 307^th Infantry) as floating reserve.

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After the US forces had taken the Gilbert and Marshall island groups in 'Galvanic', 'Flintlock' and 'Catchpole', there could be no mistaking the north-easterly axis along which the forces of Nimitz’s Pacific Ocean Areas command were advancing, and the Japanese began to reinforce the Mariana islands group, their strength on Guam rising in the short term to 11,500 men. Lieutenant General Takeshi Takashina’s 29^th Division (18^th, 38^th and 50^th Regiments) arrived on Saipan from Manchukuo via Japan in February 1944 and, less the 50^th Regimentwhich was allocated to Tinian, moved to Guam during the following month. Almost half of the 18^th Regiment had been lost when its transport vessel was torpedoed en route from Japan, but the regiment was rebuilt and its 1/18^th Regiment remained on Saipan. The division doubled as the Southern Marianas Army Group responsible for the defence of Guam and Rota, the small island between Guam and Tinian, and Tinian was transferred to its command on 7 July before the fall of Saipan.

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A reinforcement unit, the 6^th Expeditionary Unit, arrived in April and was reorganised into Major General Kiyoshi Shigematsu’s 48^th Independent Mixed Brigade (four battalions) and the 10^th Independent Mixed Regiment less its 1^st Battalion which was deployed on Rota. There were also anti-aircraft units and three tank companies with between 33 and 38 tanks.

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Under the command of Captain Yutaka Sugimoto, the Imperial Japanese navy personnel on the island totalled some 7,600 men of the 54^th Guard Force, 60^th Anti-Aircraft Defence Unit and marooned sailors and air service troops organised for ground combat.

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The Japanese concentrated most of the 38^th Regiment on Agat Bay, the 54^th Guard Force on the Orote peninsula, part of the 18^th Regiment near Apra Harbor, and most of the 48^th Independent Mixed Brigade at Agana Bay. Elements of the 10^th Independent Mixed Regiment were positioned around the south and east coasts.

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Also on the island was Lieutenant General Hideyoshi Obata, commanding the 31^st Army tasked with the defence of the entire Mariana islands group. Obata had returned from an inspection tour of the Japanese forces in the Palau islands group on learning of the US invasion of Saipan, and then established his new headquarters on Guam. While supervising the overall defence of the Mariana islands group, Obota left tactical comman