'Dynamo' was the British and to a limited extent French evacuation of British, French and some Belgian troops from the pocket on the Franco-Belgian north coast between Dunkirk in the west and De Panne in the east during the initial collapse of France following 'Sichelschnitt' (26 May/4 June 1940).

In this pocket were trapped General the Lord Gort’s British Expeditionary Force and elements of Général d’Armée George Maurice Jean Blanchard’s (from 26 May Général de Corps d’Armée René Jacques Adolphe Prieux’s) French 1ère Armée, pressed on the eastern side by Generaloberst Fedor von Bock’s Heeresgruppe 'B' and to a lesser extent on the south-western side by Generaloberst Gerd von Rundstedt’s Heeresgruppe 'A' after the German forces had split the Allied forces into two by reaching the sea at Noyelles at the mouth of the Somme river on 20 May. By 21 May, the German forces had therefore trapped the British Expeditionary Force, the remnants of the Belgian forces, and three French armies in an area along the north coast of France.

On 19 May, Gort had met with French Général d’Armée Gaston Henri Gustave Billotte, the commander of the French 1er Groupe d’Armées and overall co-ordinator of the Allied forces in north-eastern France, who had then revealed that the French had no troops between the Germans and the sea. Gort was quick to appreciate that evacuation across the English Channel was the best course of action now available to him, and began planning for the BEF’s withdrawal to Dunkirk, the closest location with good port facilities. Surrounded by marshes, Dunkirk possessed old but sturdy fortifications and the longest sand beach in Europe, where large groups could assemble. The British withdrawal to this area was aided by the Germans' controversial decision to halt, with Adolf Hitler’s approval, on 22 May. This gave the trapped Allied forces time to improve existing and construct new defensive works, and to pull back large numbers of troops toward Dunkirk. From 28 to 31 May 1940, in the siege of Lille, the remaining 40,000 men of the French 1^st Army fought a delaying action against seven German divisions, including three Panzer divisions.

Thereafter increasing German armoured pressure was exerted on the Allied forces to the north-east of the German corridor as the German infantry divisions consolidated along the line of the Somme river. The German strategic plan was centred on the elimination of the Allied forces in the north before turning their attention to the French armies in the south, but Hitler was worried about the dangers of committing the all-important German armour to the wet and therefore soft terrain of northern France, and readily accepted the offer of Generalfeldmarschall Hermann Göring that his Luftwaffe could crush the Allied forces while the army merely contained them.

Without informing the French, the British began the planning process for 'Dynamo', the seaborne evacuation of the BEF, on 20 May. The operation’s codename was suggested by the dynamo room which provided electricity in the naval headquarters below Dover Castle. It was in this room that Vice Admiral B. H. Ramsay planned the operation and briefed Churchill as it was under way. Under Ramsay’s direction, there began the gathering of ships and other craft for the evacuation. On 20 May the BEF sent Colonel G. H. P. Whitfield, the assistant adjutant general, to Dunkirk to start evacuating 'useless mouths'. Immediately overwhelmed by the size and chaotic nature of the retreat, Whitfield decided that he had no option but to send many back without thoroughly checking their credentials, and even officers instructed to stay behind to aid the evacuation disappeared onto the boats.

On 22 May Churchill ordered that the BEF should attack to the south in co-ordination with the French 1^st Army to re-establish a land link with the rest of the French forces in the so-called 'Weygand Plan', named for Général d’Armée Maxime Weygand, appointed supreme commander after the dismissal of Général d’Armée Maurice Gustave Gamelin on 18 May. On 25 May Gort had to abandon any hope of achieving this objective, and on his own authority withdrew, along with Blanchard’s forces, behind the Lys Canal, part of a canal system that reached the sea at Gravelines and thus shielded the western approaches to the selected embarkation area. The sluice gates had already been opened all along the canal to flood extra water into the system to create the 'Canal Line' barrier against the German advance.

The British and French retreat was undertaken in chaotic conditions: abandoned vehicles blocked every road, and the presence of very large and inchoate numbers of refugees further complicated military movement. As a result of censorship and the desire not to disturb British morale, the full extent of the unfolding disaster at Dunkirk was not initially publicised.

Just before 19.00 on 26 May Churchill ordered the formal start of 'Dynamo'. By this point some 28,000 men had already departed by sea. The initial plan called for the evacuation of 45,000 of the BEF’s men within two days, at which time the German advance was expected to block further evacuation. Only 25,000 men escaped during this period, including 7,669 on the first day.

By 24 May the Germans had captured the port of Boulogne and surrounded the port of Calais. Engineers of Generalleutnant Rudolf Veiel’s 2^nd Panzerdivision within General Heinz Guderian’s XIX Corps (mot.) established five bridgeheads over the Canal Line, and only a single British battalion blocked the Germans from seizing the port facilities at Dunkirk. It was at this point, at the urging of von Rundstedt, the commander-in-chief of Heeresgruppe 'A', and Göring, that Hitler issued an order halting the German armoured formations. Though some German commanders attempted to lay the blame for this operationally disastrous halt order exclusively on Hitler, it was in fact von Rundstedt who was the primary mover as he was concerned about the vulnerability of his flanks, the delivery of supplies to his forward troops, the the fact that the marshy nature of the ground around Dunkirk would be unsuitable for the employment of armour, and his desire to maintain his armoured strength at as high a level as possible for the imminent advance on Paris. These concerns were shared by Hitler, who merely validated the order several hours after the fact.

Göring also urged on Hitler the suggestion that his Luftwaffe, aided by the continued advance of von Bock’s Heeresgruppe 'B' from the east, could complete the defeat of the British. One of many senior officers concerned by Göring’s overblown belief in the Luftwaffe’s capacity to bring about a decisive victory was General Franz Halder, chief of the general staff of the Oberkommando des Heeres, who noted that the effective use of air power was wholly dependent upon the weather. Moreover, the German aircrews were worn out after two weeks of battle. The halt order was sent in clear, and picked up by the British: 'By order of the Führer…attack to the north-west of Arras is to be limited to the general line Lens-Béthune-Aire-St Omer-Gravelines. The canal will not be crossed.'

Heeresgruppe 'B' and the Luftwaffe were unable to complete the mission entrusted to them as a result of the strong British fighter presence in the area, the lack of motorised transport which slowed the advance of the infantry, and adverse weather. On 26 May, Hitler ordered the Panzer formations to resume their advances, but the delay had provided the British with the time to construct the defences vital to the evacuation.

On 27 May, the first full day of the evacuation, one cruiser, eight destroyers and 26 other vessels were involved even as Admiralty officers combed nearby boatyards for small craft able to ferry personnel from the beaches out to larger craft in the harbour, as well as larger vessels that could load from the docks. An emergency call was put out for additional help, and by 31 May nearly 400 small craft were voluntarily and enthusiastically taking part in the effort.

Also on 27 May, the Luftwaffe heavily bombed both the town and the dock installations of Dunkirk, and the destruction of the area’s water supply meant that the fires resulting from the bombing could not be extinguished. It is estimated that about 1,000 civilians were killed, this representing about 33% of the town’s remaining population. The Luftwaffe was met by 16 British squadrons, whose pilots claimed 38 kills on 27 May for the loss of 14 British aircraft. Throughout 'Dynamo', the British flew more than 3,500 sorties. The RAF continued to exact a heavy toll from the German bombers throughout the week. Soldiers being bombed and strafed while awaiting transport were for the most part unaware of the efforts of the RAF, as most of the dogfights took place far from the beaches, and as a result, many British soldiers bitterly accused the air force of doing nothing to help.

On 25 and 26 May, the Luftwaffe concentrated its efforts on Allied pockets holding out at Calais, Lille and Amiens, and did not attack Dunkirk. Held by the BEF, Calais surrendered on 26 May. The remnants of the French 1^st Army, surrounded at Lille, fought off seven German divisions until 31 May, when the remaining 35,000 men were forced to surrender for lack of food and ammunition. The Belgian army surrendered on 28 May, leaving a large gap in the protective shield to the east of Dunkirk: several British divisions were rushed to cover that side of the perimeter. On 30 May, Churchill received word that all British divisions, together with more than half of the French 1^st Army, were behind the defensive perimeter. By this time this perimeter extended along a series of canals about 7 miles (11.25 km) from the coast, in marshy terrain unsuitable for tank warfare.

With the docks in the harbour rendered unusable by German air attacks, Captain W. G. Tennant, the senior naval officer, initially ordered men to be evacuated from the beaches, but when this proved to be too slow a process, he rerouted the troops to a pair of long stone and concrete breakwaters, called the East and West Moles, as well as the beaches. Almost 200,000 troops embarked on ships from the East Mole, which extended almost 1 mile (1.6 km) out to sea, in the course of the next week.

On 28 May, the day on which the area held by the Allied forces was reduced to 11.6 sq miles (30 km²), 17,804 soldiers arrived at British ports, on the following day 47,310 British troops were rescued, and on 30 May 53,823 men, including the first French soldiers, were embarked. Gort and 68,014 men were evacuated on 31 May, and Major General the Hon. Harold Alexander, commanding officer of the 1^st Division, was left in command of the rearguard. Another 64,429 Allied soldiers departed on 1 June before the increasing strength of the German air attacks prevented further daylight evacuation. The British rearguard of 4,000 men left on the night of 2/3 June, together with 60,000 French soldiers, and another 26,000 French troops were retrieved on the following night before 'Dynamo' ended. The rest of the rearguard, comprising 40,000 French troops, surrendered on 4 June.

The final tally of those evacuated was 338,226 men, of whom 98,671 were lifted from the beaches and the other 239,555 men from the harbour.

The ships and craft used in 'Dynamo' operated along three routes. The shortest of these was Route Z, a distance 45 miles (72.25 km), but this entailed hugging the French coast and thus the vessels and craft using it were subjected to bombardment from artillery on the coast, especially in the daylight hours of this mid-year period. Route X, although the safest from attack by coastal artillery, travelled through a particularly heavily mined portion of the English Channel: ships and craft on this route travelled 63.25 miles (101.75 km) north out of Dunkirk, proceeded through the Ruytingen Pass, and headed toward the North Goodwin Lightship before turning south around the Goodwin Sands to Dover. This route was safest from surface attacks, but the nearby minefields and sand banks meant it could not used at night. The longest of the three was Route Y, a distance of 100 miles (161 km). Use of this route increased the sailing time to four hours, double the time of Route Z. The route followed the French coast as far as Bray Dunes, then turned to the north-east until reaching the Kwinte Buoy where, after making an almost 270° turn, the ships sailed west to the North Goodwin Lightship and then headed south around the Goodwin Sands to Dover. It was the ships and craft on Route Y which were most likely to be attacked by German surface vessels, U-boats and aircraft.

For 'Dynamo' the Royal Navy provided the light anti-aircraft cruiser Calcutta, 39 destroyers, and many other craft. The British merchant marine supplied passenger ferries, hospital ships, and other vessels, and the Belgians, Dutch and French also provided a miscellany of other vessels. Ramsay arranged for about 1,000 copies to be made of the required charts, had buoys laid around the Goodwin Sands and down to Dunkirk, and organised the flow of shipping. Larger ships such as destroyers were able to carry about 900 men per trip. The soldiers travelled mostly on the upper decks to minimise losses if the vessel was sank.

After the loss on 29 May of 19 British and French naval vessels and three of the larger requisitioned ships, the Admiralty withdrew the eight best British destroyers from the operation to ensure they would be available for the future defence of the country.

'Dynamo' thus involved the use of one cruiser, which was damaged, 39 destroyers of which six were sunk and 19 damaged, nine sloops, corvettes and gunboats of which one was sunk and one damaged, 36 minesweepers of which five were sunk and seven damaged, 113 trawlers and drifters of which 17 were sunk and two damaged, three special service vessels of which one was sunk, three ocean boarding vessels of which one was sunk and one damaged, 13 torpedo boats and anti-submarine boats, 40 naval-manned ex-Dutch schuyts of which four were sunk and an unknown number damaged, 26 naval-manned yachts of which three were sunk and an unknown number damaged, 45 personnel ships of which eight were sunk and eight damaged, eight hospital carriers of which one was sunk and five damaged, 12 naval motor boats of which six were sunk and an unknown number damaged, 34 tugs of which three were sunk and an unknown number damaged, and 311 other small craft (excluding ships' lifeboats and some small private craft which were not recorded) of which 170 were sunk and an unknown number damaged.

In addition to Calcutta, the British warships involved in 'Dynamo' were the destroyers Anthony, Basilisk, Codrington, Esk, Express, Gallant, Grafton, Grenade, Greyhound, Harvester, Havant, Icarus, Impulsive, Intrepid, Ivanhoe, Javelin, Jaguar, Keith, Mackay, Malcolm, Montrose, Sabre, Saladin, Scimitar, Shikari, Vanquisher, Venomous, Verity, Vimy, Vivacious, Wakeful, Whitehall, Whitshed, Wild Swan, Winchelsea, Windsor, Wolfhound, Wolsey, Worcester and Free Polish Błyskawica, sloops Bideford, Guillemot and Kingfisher, gunboats Locust and Mosquito, minesweepers Albury, Dundalk, Gossamer, Halcyon, Hebe, Leda, Lydd, Niger, Pangbourne, Ross, Salamander, Saltash, Skipjack, Speedwell, Sutton and Sharpshooter, four patrol ships, 53 minesweeping and anti-submarine trawlers, five Q-ships, 24 drifters, six motor torpedo boats, four anti-submarine motor boats, 28 personnel transports and eight hospital transports, as well as numerous auxiliary and private craft. In addition, the French destroyers Epervier, Léopard, Bourrasque, Cyclone, Foudroyant, Mistral and Siroco, torpedo boats Branlebas, Boucher, Flore and Incomprise, and sloops Amiens, Amiral Mouchez, Arras and Belfort, as well as auxiliary and merchant ships, were involved.

Thus to the British component were added 49 Allied warships of all types, of which eight were sunk and an unknown number damaged, and 119 other Allied ships of which nine were sunk and an unknown number damaged. In overall terms, therefore, 168 Allied vessels, of which 17 were sunk, complemented 693 British vessels, of which 226 were sunk, for an overall total of 861 ships and craft, of which 243 were sunk.

Six British and three French destroyers were sunk, along with nine other major vessels. In addition, 19 destroyers were damaged. More than 200 British and Allied craft were sunk, with a similar number damaged. The most significant British losses were the destroyers Grafton sunk by U-62 on 29 May, Grenade sunk by air attack at Dunkirk on 29 May, Wakeful sunk by a torpedo from the S-boot S-30 on 29 May, and Basilisk, Havant and Keith sunk by air attack off the beaches on 1 June. The most significant French losses were the destroyers Bourrasque mined off Nieuport on 30 May, Sirocco sunk by the S-boote S-23 and S-26 on 31 May, and Foudroyant sunk by air attack off the beaches on 1 June.

It was on 27 May that a request was placed to civilians to provide all shallow-draught vessels of 30 to 100 ft (9 to 30 m) for the operation. The great miscellany of small vessels from all over the south of England pressed into service included speedboats, Thames vessels, car ferries, pleasure craft, and many other types of small craft, some of them requisitioned without their owners' knowledge and consent. The most useful proved to be the motor lifeboats, which had a reasonably good capacity and speed. Agents of the Ministry of Shipping, accompanied by a naval officer, scoured the Thames river for possible candidate vessels, had them checked for seaworthiness, and took them downriver to Sheerness in Kent, where they were allocated naval crews, but as a result of the shortage of personnel many small craft crossed the English Channel with civilian crews.

The first of these 'little ships' reached Dunkirk on 28 May. Dunkirk’s wide sand beaches meant that larger vessels could not get anywhere near the shore, and even small craft had to stop about 100 yards (91 m) from the waterline and wait for the soldiers to wade out. In some cases personnel abandoned their craft upon reaching a larger ship, and subsequent evacuees had to wait for these craft to drift ashore with the tide before they could make use of them. In most beach areas, soldiers queued with their units and patiently awaited their turn to leave, but on some occasions panicky soldiers had to be warned off at gunpoint when they attempted to rush to the boats out of turn. In addition to being ferried on boats, soldiers at De Panne and Bray Dunes constructed improvised jetties by driving rows of abandoned vehicles onto the beach at low tide, anchoring them with sandbags, and connecting them with wooden walkways.

The BEF lost 68,000 men dead, wounded, missing or captured between the German invasion of 10 May and the French surrender of 22 June. All the heavy equipment had to be abandoned, so left in France were 2,472 pieces of artillery, 20,000 motorcycles and almost 65,000 other vehicles, as well as were 371,430 tons of stores, more than 67,000 tons of ammunition and 144,445 tons of fuel. Almost all of the 445 British tanks that had been sent to France with the BEF were abandoned.

The RAF lost 145 aircraft, of which at least 42 were Supermarine Spitfire fighters, while the Luftwaffe lost 156 aircraft in operations in the nine days of 'Dynamo'. British warships claimed the destruction of 35 Luftwaffe aircraft with gunfire during the period from 27 May to 1 June, and damage to another 21 aircraft. Aircraft losses from 10 May until the fall of France were 959 for the British and 1,279 for the Germans.

Three British divisions and a large numbers of logistic and labour troops had been isolated to the south of the Somme river by the initial German 'race to the sea'. At the end of May, a further two divisions began moving to France with the hope of establishing a 2^nd BEF. Most of Major General V. M. Fortune’s 51^st Division was forced to surrender on 12 June, but almost 192,000 Allied personnel, 144,000 of them British, were evacuated through various French ports in the period 15/25 June in 'Ariel'.

On arrival in the UK, more than 100,000 evacuated French troops were quickly transported to camps in various parts of south-western England, where they were temporarily lodged before being repatriated. British ships ferried French troops to Brest, Cherbourg and other ports in Normandy and Brittany, although only about half of the repatriated troops had been deployed against the Germans before the surrender of France. For many French soldiers, the Dunkirk evacuation represented only a delay of only a few weeks before being killed or captured by the German army after their return to France. Of the French troops evacuated from France in June 1940, only about 3,000 joined Général de Brigade Charles de Gaulle’s Free French army in the UK.

For every seven men who escaped through Dunkirk, one was left to become a prisoner of war. The majority of these prisoners were sent on forced marches into Germany. The majority of these prisoners of war were then put to work in German industry and agriculture for the remainder of the war.

The rescue of the British troops at Dunkirk provided a psychological boost to British morale which ended any possibility that the UK would seek peace terms from Germany now that it still retained (it was believed) the ability to defend the country against a possible German invasion.

In France, however, a perception that the Royal Navy had prioritised the evacuation of British forces at the expense of French forces led to a degree of bitter resentment.