As early as June 20, Darlan had sent to all ship and port commanders encrypted instructions which contained this passage:“Whatever orders received, never abandon an intact vessel to the enemy. Four days later, after the signing of the armistice, he confirmed these orders, adding to them that of preparing everything to scuttle the ships in case they were in danger of being captured. Yet, even if these directives had been known in London, they would not have changed the belief of the British cabinet that if the Germans planned to seize the ships, they would succeed.
Accordingly, the June 27, it was decided to take measures to prevent French ships from returning to metropolitan ports. The following day, Vice-Admiral Sir James Somerville was given command of the newly formed H Force, consisting of the battlecruiser Hood (flagship), the battleships Resolution and Valiant, the aircraft carrier Ark Royal, two cruisers and several destroyers. Based in Gibraltar, she was well placed to operate in the Atlantic, in support of the Home Fleet, or in the Western Mediterranean, which would otherwise have been at the mercy of the Italian fleet.
On July 1, Somerville received his first mission:to ensure the transfer to English ports, the surrender or the destruction of the French buildings anchored in the base of Mers el-Kébir, near Oran. Force H was to report there on July 3, at daybreak, to offer Admiral Gensoul the choice between four solutions:join the British fleet to continue the fight; depart with reduced crews for British ports, or the West Indies; disarm ships at Mers el-Kébir under British control; scuttle them on the spot. If none of these solutions were accepted, force would be used to destroy the buildings.
Many British naval officers were opposed to an action of force against a disciplined navy which, only the day before, had taken its part in the struggle against the common enemy. Somerville was impressed by the unanimity of the views of his staff both on the need to avoid the threat of acting by force and on the fact that no French admiral, faced with such an ultimatum, could do otherwise than to defend oneself. Having reported these views to the Admiralty, he received the following reply that if the French refused, "It was the firm intention of His Majesty's Government to destroy the buildings".
Arrived in front of Oran in accordance with the plans, Somerville intended to bring Captain Holland, former naval attache in Paris, into the port, so that he could present the ultimatum to Admiral Gensoul in nobody aboard his flagship, the Dunkirk. But, Admiral Gensoul having refused to receive Commander Holland, the ultimatum was sent in writing, through a French emissary. At 10 a.m., Admiral Gensoul replied by message that he would not be the first to open fire, but that force would respond to force. Informing Darlan by radio of what was happening, he mentioned only that he had received a British ultimatum, advising him that his buildings would be sunk within six hours; at the same time he ordered his ships to put on fires and prepare for battle.
After the exchange of several messages transported by boat, Admiral Gensoul, with no response from Darlan, accepted to have Holland on board, to save time. Only then does Holland live; for the first time, the orders given by Darlan ten days earlier to prevent the capture of the French ships. In the tense atmosphere that reigned over Dunkirk, the talks were not facilitated by the indignation of Admiral Gensoul, reacting to the mining of the port carried out at the beginning of the afternoon by the planes of the Ark Royal .
In the meantime, Admiral Le Luc, Chief of the Naval Staff, acting on behalf of Darlan, who could not be reached, had informed Gensoul that all French naval forces in the Mediterranean had received the order to sail to lend him a hand. This message was intercepted by the Admiralty in London, who then immediately ordered Somerville to settle the matter without delay.
Somerville had already exceeded his deadlines,' and he informed Gensoul that if he had not accepted the 5:30 p.m. ultimatum, his ships would be sunk. Five minutes before the deadline, Holland left Dunkirk and returned to his ship.
Shortly before 6 p.m., Somerville opened fire on the French ships, embarrassed to return fire, as they were in the process of spin their cables and maneuver to reach the sea.
Fired from a distance of about 13,000 meters, the salvoes of the 375 mm guns of the English soon reached their goals. The battleship Bretagne was hit first. Two new hits caused a terrible explosion and, in a huge cloud of smoke, the battleship capsized; 977 men perished.
Several shells hit the Dunkirk and the vessel had barely enough steam to drop anchor at the bottom of the harbour. The Provence had moved away from the quay and had fired three salvoes at the Hood when a shell, reaching its rear turret, set fire to the ammunition hold, which had to be flooded. Further damage forced her to run aground to avoid sinking.
The destroyer Mogador, her bow torn off as she tried to set sail with four other destroyers, lost 37 men and her burning wreckage had to be towed to port. Meanwhile, the second battlecruiser, the Strasbourg, had taken advantage of the cloud of smoke from the Bretagne to escape to the high seas.
Thirteen minutes after the start of the attack, the Admiral Gensoul, desiring to avoid unnecessary bloodshed, tried to signal Somerville to cease fire. Fortunately, the British admiral, who personified the unanimous disgust of the Royal Navy for such an operation, did not wish to prolong the massacre any longer and he decided to break off the engagement. As mines had been laid at the entrance to the port, he did not expect to see French ships escaping. But when he learned that the Strasbourg had been able to reach the high seas without damage and was heading east, he chased it and had it attacked by the "Swordfish" of the Ark Royal. At 8:20 p.m., the Strasbourg was already 25 miles ahead and the English gave up pursuing it after a second, unsuccessful air attack. The Strasbourg, the aircraft carrier Commandant-Teste, five destroyers and the six cruisers from Algiers arrived in Toulon on the evening of July 4, and Force H returned to its base, Gibraltar.