'Catapult' was a British attempt to destroy major elements of the French navy at Mers el Kébir, just outside Oran in the French North African territory of Algeria, to remove the possibility of France’s surviving major warships (Vice-amiral d’escadre Marcel-Bruno Gensoul’s Force de Raid recently transferred from Brest) falling into German hands after France’s June 1940 armistice with Germany (3 July 1940).

The attack on Mers el Kébir was the single most important part of 'Catapult'. In what is otherwise known as the Battle of Mers el Kébir, warships of a British task force attacked the element of the French navy in its base at Mers el Kébir on the coast of French Algeria after giving the French an explicit warning that this was about to happen. The French ships were at anchor and had not anticipated any attack by British ships given the fact that France and the UK until little more than a week earlier. The attack resulted in the deaths of 1,297 French servicemen, the sinking of a battleship and the damaging of five other ships.

France and the UK were not at war, but on 26 June a French armistice with Germany came into effect and the British feared the French fleet would be absorbed into the German navy, an event which would greatly increase German naval strength and significantly alter the naval balance of power between the UK and Germany. Although the commander-in-chief of the French navy, Amiral de la flotte François Darlan, had assured Prime Minister Winston Churchill that the French fleet would not be allowed to fall into German hands, the British believed that they had to act on the assumption that Darlan could not ensure this eventuality.

The new government of what was now Vichy France did not retaliate to 'Catapult', and for the rest of the war maintained a state of armed neutrality. The attack was always controversial, not least within the Royal Navy, and created much rancour between Vichy France and the UK. It also clearly demonstrated to the world in general, and to the USA in particular, that the UK was still totally committed to a continued war with Germany at all costs and, of need be, without allies.

With the implementation of the Franco-German armistice in June 1940, the UK became very concerned about the possibility of the powerful French fleet falling into German hands, for a combination of French and German navies could tip the balance of naval power toward Germany, thereby threatening the UK’s ability to ship raw materials across the Atlantic, and also its ability to maintain maritime communications with the rest of its empire. There were both possibilities which Churchill’s coalition government feared despite the fact that the armistice terms included the statement that the German government 'solemnly and firmly declared that it had no intention of making demands regarding the French fleet during the peace negotiations', and that similar terms existed in the French armistice with Italy. In this fear the British were being no more than realistic, for they already knew to their cost how readily Adolf Hitler was prepared to break his word. Furthermore, on 24 June, Darlan had assured Churchill that he would ensure that the French fleet would not fall in German hands. There was no way for the French to ensure this, although it should be noted that when the Germans attempted 'Lila' during November 1942 to seize the French naval strength in Toulon, in direct contravention of the armistice terms, some French ships eacaped to join the others while those which could not were scuttled by their crews.

In June 1940 one of Churchill’s responses to the Franco-German armistice was a demand that the French navy should either join forces with the Royal Navy, or be neutralised in some way to prevent the ships from falling into German or Italian hands. In a speech to parliament, Churchill repeated that the Franco-German armistice was a betrayal of the Allied agreement which forbade each country from surrendering to the Germans without notifying its allies. This French betrayal, added to the Nazi government’s history of failing to respect previous agreements, led Churchill to declare 'What is the value of that? Ask half a dozen countries, what is such a solemn assurance? Furthermore, the armistice could be voided at any time on the pretext of ''non observance''.'

As of 26 June the British were already faced with a situation in which France’s English Channel and Atlantic ports and naval bases were in German hands, and now on their own had to keep the German navy out of the Mediterranean, to lock the Italian navy into the Mediterranean, and to blockade Vichy French ports. Moreover, to keep the UK’s Atlantic approaches open to convoy traffic, the British lacked sufficient numbers of warships with which to undertake a permanent blockade of Vichy France’s North African and West African naval bases. In such circumstances, the risks of making it possible for either the Germans or the Italians seize the major French warships was too great for the British to contemplate.

At this time the French fleet was widely dispersed. Some of its strength was in the bases of metropolitan France, some in the bases of other parts of the French empire, and modest numbers of smaller warships had escaped from France to British-controlled ports, mainly in the UK and at Alexandria in Egypt.

'Catapult' was now created to ensure that the warships in Vichy French hands were taken into British control or, if this proved impossible, destroyed. In the first stage, the French ships in the British ports of Plymouth and Portsmouth were simply boarded ib 'Grasp' during the night of 3/4 July. The only resistance came from the French submarine cruiser Surcouf, then the largest submarine in the world, which had sought refuge in Portsmouth in June 1940 following the German invasion of France. The submarine’s crew resisted the boarding, and three British personnel, including two officers, were killed; one French sailor was also killed. Other ships captured included the two obsolete battleships Paris and Courbet, the destroyers Triomphant and Léopard, eight torpedo boats, five submarines and a number of less important ships. Many, including Surcouf went on to be used by the Free French forces. Some sailors joined the Free French while others were repatriated to France. It is also worth noting that the attack sowed anger and resentment among the French toward the British, and further heightened the tension between Churchill and the leader of the Free French forces, Général de Brigade Charles de Gaulle.

Late in June 1940, the most powerful concentration of French warships was the squadron in the base at Mers el Kébir. This force comprised the World War I era battleships Provence and Bretagne, the more modern fast battleships Dunkerque and Strasbourg, the seaplane tender Commandant Teste and six destroyers under the command of Gensoul.

Vice Admiral Sir James Somerville, commanding the Gibraltar-based Force 'H', was ordered to deliver an ultimatum to the French, stating:

'It is impossible for us, your comrades up to now, to allow your fine ships to fall into the power of the German enemy. We are determined to fight on until the end, and if we win, as we think we shall, we shall never forget that France was our Ally, that our interests are the same as hers, and that our common enemy is Germany. Should we conquer we solemnly declare that we shall restore the greatness and territory of France. For this purpose we must make sure that the best ships of the French Navy are not used against us by the common foe. In these circumstances, His Majesty’s Government have instructed me to demand that the French Fleet now at Mers el Kebir and Oran shall act in accordance with one of the following alternatives:

'(a) Sail with us and continue the fight until victory against the Germans.

'(b) Sail with reduced crews under our control to a British port. The reduced crews would be repatriated at the earliest moment.

'If either of these courses is adopted by you we will restore your ships to France at the conclusion of the war or pay full compensation if they are damaged meanwhile.

'(c) Alternatively if you feel bound to stipulate that your ships should not be used against the Germans unless they break the Armistice, then sail them with us with reduced crews to some French port in the West Indies – Martinique for instance – where they can be demilitarised to our satisfaction, or perhaps be entrusted to the United States and remain safe until the end of the war, the crews being repatriated.

'If you refuse these fair offers, I must with profound regret, require you to sink your ships within 6 hours.

'Finally, failing the above, I have the orders from His Majesty’s Government to use whatever force may be necessary to prevent your ships from falling into German hands.'

Somerville did not present the ultimatum personally, this highly distasteful task instead falling to the French-speaking Captain Cedric Holland, commanding the fleet carrier Ark Royal. Affronted by the fact that negotiations were not being conducted by a more senior officer, however, Gensoul sent his flag lieutenant to meet Holland, and this led to much delay and confusion. As negotiations continued, it became clear that neither side was likely to concede. Now the Vichy French navy minister, Darlan did not receive the full text of the British ultimatum from Gensoul, most significantly that part offering the option of a removal to US waters: this option was in fact part of the orders which Darlan had given to Gensoul for implementation should any foreign power attempt to seize the ships under his command.

The British force which sailed for Mers el Kébir comprised the battle-cruiser Hood, the battleships Valiant and Resolution, the fleet carrier Ark Royal, the light cruisers Arethusa and Enterprise, and the destroyers Active, Escort, Fearless, Foresight, Forester, Foxhound, Keppel, Vidette, Vortigern and Wrestler.

The British and French forces were about equal in matériel terms, but the British nonetheless had several decisive advantages. The French fleet was anchored in a narrow harbour and, despite the unequivocal terms of the ultimatum, did not expect an attack and was not fully prepared for battle. The eight 12.99-in (330-mm) main armament guns of Dunkerque and Strasbourg were grouped in the forward parts of the ships in quadruple turrets and could not immediately be brought to bear. The British capital ships, with 15-inch (381-mm) guns, also fired a heavier broadside than the those of the French ships.

Before the end of negotiations, Fairey Swordfish attack aircraft escorted by Blackburn Skua dive-bomber/fighters were launched from Ark Royal to drop magnetic mines in the path the French ships would have to use should they try to put out to sea. The aircraft were intercepted by French Curtiss Hawk H75 fighters, which shot down one of the Skua aircraft that went down into the sea with the loss of its two-man crew, the only British fatalities in the action.

Soon after this, and on Churchill’s instructions with the full support of Admiral Sir Dudley Pound, the First Sea Lord, the British ships began the action when they opened fire at 17.54 on 3 July at extreme range. Provence started to return the British fire, but only after 90 seconds, while Dunkerque and Strasbourg were unable to fire at once because they were moored at the mole with their sterns to the sea and their main batteries therefore facing inland.

Dunkerque was hit by three shells, immobilised and lost 210 men killed, and Gensoul ordered her to be run aground. Bretagne was hit heavily and sank after suffering a magazine explosion, taking with her 1,012 of her crew. Provence was also hit several times, badly damaged and lost three of her crew. Mogador lost her stern as a result of a direct hit and lost 38 men killed.

Somerville did not realise that the boom barrage had been opened on Gensoul’s orders, so the British were taken by surprise when Strasbourg and the five remaining destroyers (Kersaint, Lynx, Le Terrible, Tigre and Volta) managed to avoid the mines and escape to the open sea at high speed. As they did so they came under attack from a flight of bomb-armed Swordfish from Ark Royal. The French ships responded with anti-aircraft fire and shot down two of the Swordfish machines, whose crews were rescued by the destroyer Wrestler, which was engaged but missed by Strasbourg.

Somerville ordered his forces to begin a pursuit at 18.43. The light cruisers Arethusa and Enterprise reported engaging a French destroyer but, at 20.20, Somerville called off the pursuit, feeling that his ships were ill-deployed for a night engagement.

Two of the French destroyers attacked the British submarine Proteus without success, while two others fired torpedoes at Hood at long range, again without success. The submarines Danae and Euryalus came under attack by aircraft and a destroyer, and could not reach a firing position on Resolution. After weathering another Swordfish attack at 20.25 without suffering any damage, Strasbourg crossed the Mediterranean to reach the French mainland base at Toulon with Le Terrible, Tigre and Volta during the evening of 4 July. The total French naval losses had been 1,300 men killed and about 350 men wounded.

On 4 July, the British submarine Pandora sank the French aviso (gunboat) Rigault de Genouilly, sailing from Oran. During the night of 4/5 July Vichy French bombers carried out a retaliatory raid against the British ships at Gibraltar to minimal effect.

As they believed that damage to Dunkerque and Provence not to be serious, the British launched Swordfish aircraft from Ark Royal against Mers el Kebir during the morning of 6 July. One air-launched torpedo hit the patrol boat Terre-Neuve, which was moored alongside Dunkerque and was carrying a supply of depth charges. Terre-Neuve quickly sank and its charges triggered a large explosion, causing further serious damage to Dunkerque.

The last phase of 'Catapult' was an attack on 8 July by aircraft of the light carrier Hermes against the French battleship Richelieu at Dakar in French West Africa.

In response to the actions at Mers el Kébir and Dakar, the Vichy French launched retaliatory bombing raids on British targets at Gibraltar, including a half-hearted attack on 14 July when many bombs landed in the sea, and heavier raids on 24 and 25 September.

As a result of 'Catapult', the Préfet Maritime at Toulon ordered the establishment of two submarine groups (Groupe 'A' with Espoir, Conquérante and Archimède, and Groupe 'B' with Iris, Vénus, Sultane, Sirène, Pallas and Cérès) to render assistance to Gensoul by attacking British ships, especially Hood. None of these boats established contact, however, and with the report that the British ships had returned to Gibraltar on 5 July the submarines were recalled.

At Dakar Le Héros and Le Glorieux were ordered to depart and intercept the British heavy cruiser Dorsetshire, which evaded the attackers by means of high-speed manoeuvring. The submarines were then recalled. Off Beirut, the submarines Dauphin and Actéon occupied defensive positions off the coast of Lebanon but returned after a few days.

The French ships in Alexandria under command of Contre-Amiral René Emile Godfroy (the old battleship Lorraine, the heavy cruisers Duquesne, Suffren and Tourville, the light cruiser Duguay Truin, the destroyers Basque, Forbin and Fortuné, and the submarine Protée) were blockaded by the British on 3 July and offered the basically same terms as those offered at Mers el Kébir. After delicate negotiations, conducted on the part of the British by Admiral Sir John Cunningham, commander-in-chief of the Mediterranean Fleet, Godfroy agreed on 7 July to disarm his ships and stay in port until the end of the war. In fact the French ships remained in Alexandria until eventually joining the Allies in 1943.

At Mers el Kébir Dunkerque, Provence and Mogador received emergency repairs and were then able to sail back to Toulon.

Early in June 1940, about 13,500 British civilians had been evacuated from Gibraltar to Casablanca in French Morocco. Following the Franco-German armistice and the attack on Mers el Kébir, the presence of these British nationals became an embarrassment to the Vichy French government, and late in June, 15 British cargo vessels arrived in Casablanca carrying 15,000 French servicemen who had been rescued from Dunkirk and were now being repatriated. Once the French servicemen had disembarked, the ships were interned until they agreed to evacuate all the British evacuees.