The French military was humiliated in the spring of 1940, with the German armies rapidly cutting off their army and the British at Dunkirk. The French government collapsed, and those remaining in power quickly signed an armistice with the Germans. Hitler allowed the French to have some measure of autonomy in the southern portion of the country, and their army and navy stationed in their colonies (primarily those in North Africa) remained under French control. But were those forces really under French control, or were they puppets of the Germans who now effectively governed all of France?
Winston Churchill certainly did not want the powerful French fleet to be handed over to the Nazis. Great Britain was barely hanging on. So, Churchill ordered the Royal Navy to sail to the French ports of Oran and Mers el Kebir in French Algeria where most of their fleet was. The Brits were under orders to invite the French to sail with them and join forces, sail to a British port and disarm, sail to a far away French port (like the West Indies or Martinique) and disarm or turn them over to the US, or scuttle their ships.
The French admiral, Marcel Gensoul, certainly was taken aback and offended. He did not take orders from the British. While he awaited orders from his superiors in Vichy France, Churchil was growing impatient. He believed he simply could not afford any more time to allow the French to fight or flee (although the French fleet was certainly not prepared at that moment to get under way or to start fighting). Churchill ordered his commander, Admiral Somerville, to "settle matters quickly."
On July 3, 1940, the Brits opened fire on the French navy which was still anchored. Huge sixteen inch shells blasted their way into French warships, making one ship, the "Bretagne" capsize and kill over 1,200 sailors. Three ships made it to the French port of Toulon; the rest were sunk or badly damaged. Later the French actually launched air raids against the British in Gibraltar in retaliation! They could not believe that their allies had killed over a thousand of their sailors!
Recruitment for De Gaule's Free French Forces in Britain and in French colonies declined rapidly after this terrible incident. However, when the Nazis did try to seize the remaining French fleet in France in 1942, Frenchment around the world rallied to the cause of De Gaule and his Free French Forces. The French did mistrust the Brits (for good reason), but their hatred for the Germans was much more and allowed them to eventually build up a force of over 1 million men who eventually liberated their country.
This incident against the French fleet soured Britain's reputation among the French for generations to come, but it clearly demonstrated to all Churchill's ruthlessness to do whatever it took to defeat the Nazis.