I believe what you're referring to is Operation Catapult, the British attempt to neutralise the French Navy after the fall of France in 1940. After the German invasion of France, much of the French Navy fled metropolitan France, for the North African colonies or the UK. While this theoretically put the majority of the French Navy outside German control, there was suspicion as to where their loyalties lay. Unlike the Army, the French Navy strongly supported the Vichy regime. If Germany ordered their Vichy puppets to hand the fleet over to them, it was likely that the French Navy would acquiesce. This would give the Germans a strong enough fleet to threaten the Royal Navy's control of the seas. As a result, the RN had to neutralise the French Navy in some way. French ships in British ports were simply seized and handed over to British crews. For the most part, this occurred peacefully, but aboard the submarine Surcouf four men (two British officers, a British seaman and a French NCO) were killed in a shootout.

A large part of the French fleet, including two of their most modern battleships, was based at the port of Mers-El-Kebir in Algeria. The British Force H, with three battleships and the carrier Ark Royal arrived off the port in early July 1940. Force H's commander, Admiral Somerville, provided the French with an ultimatum, giving them four options: join the Free French forces, hand their ships over to the Free French, disarm their ships and sail to French colonies in the Caribbean where the Germans couldn't seize them, or be sunk. Negotiations were protracted, but eventually broke down. The British opened fire just before 6 pm on the 3rd July, sinking or heavily damaging most of the French ships present at the port. Five days later, aircraft from HMS Hermes attacked the battleship Richelieu at Dakar, causing heavy damage.