The World War 2 Royal Navy Assault on the French Navy
In 1940, Nazi Germany stormed through Western Europe, defeated the French army and pushed the British Expeditionary Force back across the English Channel. The French leadership quickly agreed to an armistice with Germany, and France was subsequently occupied. A new seat of French government at Vichy became a puppet state of Germany, and later outright collaborators.
The United Kingdom was now alone in their fight against Germany, and feared the French Naval fleet in French African colonies would be handed over to Germany and Italy. Winston Churchill and the British Admiralty formed a plan to deliver an ultimatum to Vichy: Join the fight against Germany, sail to colonial ports in North America, sail to United States ports, sink your own ships where they are, or the Royal Navy would sink them. The Vichy French refused, and these former allies suddenly became bitter enemies.
Sources
Fiderlein, Brian, “The morality issue: How Darlan influenced the Allied unconditional surrender pledge in World War II” (1998). Dissertations and Theses @ UNI. 655. https://scholarworks.uni.edu/etd/655
Roskill, Captain S.W. “The War at Sea 1939-1945, Volume 1, The Defensive.” Naval and Military Press. 2004.