Operation Catapult

July 3, 1940: Winston Churchill faces a momentous decision. He sends British naval forces to the port of Mers-el-Kabir, where French ships are stationed. He commands the British forces to demand the surrender of the French vessels, with the threat of sinking them as a worst-case scenario.
The British were supposed to offer the French 4 different alternatives so as not to let their ships fall into Nazi hands, which was of utmost importance to their success in the war.
Churchill desperately wanted to avoid an Anglo-French battle and worked with the British War Cabinet to find a diplomatic solution to their problem. Their hope that there would be a legitimate assurance from the French that their ships would be kept from the Germans was extinguished, pushing Britain to cross the line they so didn't want to. Their negotiations with the French resulted in failure, leaving Churchill to resort to the use of force to prevent the ships from falling into Nazi hands.
Churchill weighed both options- attacking the French ships would incur the wrath of all of France, but letting them fall into German control would put the British at risk, considering the ships would fall into "Axis possession". This left Churchill with one option only; force the surrender of the ships, even at the cost of having to sink them.
The naval power of France was extreme, with seven battleships, 19 cruisers, 71 destroyers, as well as 76 submarines. After France was invaded, many of its ships sailed to foreign harbors, which is why such a powerful naval force was stationed at Mers-el-Kabir, which is what led it to have such a powerful importance. Marcel Gensoul commanded the French ships at Mers-el-Kabir. Gensoul was loyal to Jean Francois Darlan, as well as to the Vichy government (which was like a "puppet" government under German control". Marcel's beliefs led the British to believe that finding a compromise with him would be extremely difficult.
On the 27th of June, 4 options to give Gensoul were proposed. The first was to have the fleet join the Royal Navy, another was to transport the fleet (with smaller crews) to British ports, to take the fleet to a U.S. port, or, if all the previous options were declined, to sink the ships then and there.
Sir James Somerville was tasked with Operation Catapult, given Force H to aid him in his objective. He was briefed on the 29th of June, and arrived at Mers-el-Kebir on July 3rd at 5:30 am.
However, Gensoul refused to meet with Captain Holland, who had been instructed to explain the British terms to him. Instead, he sent Lieutenant Antoine Dufay to conference with Holland.
Holland gave Dufay the written British demands to give to Gensoul, but the demands merely angered Gensoul. At 9:45 am, he signaled the French Admiralty that a British force had come to Mers-el-Kabir, and had given him an ultimatum to sink his ships within six hours. Gensoul also stated that he intended to fight fire with fire, meaning that he would attack the British.

When it became obvious to the British that the French intended on leaving the Harbor, Holland ordered to have the harbor entrance sealed with mines to prevent their escape. At 5:30, Force H opened fire on the French, sinking numerous ships and killing over 1,000 men. The British gave a short reprieve to the French in order to allow them to abandon ship to prevent further loss of life, but the French used it to prepare to leave. The French were fired upon once more, and they surrendered shortly thereafter.
The attack on Mers-el-Kabir led the Vichy government to give up all diplomatic ties to London, but the materials and ships recovered by the British more than made up for the loss of their puppet ally.