During the closing days of the Battle of France in 1940, and with the German victory all but assured, the British became increasingly worried about the disposition of the French fleet.
The French navy was the fourth largest in the world at this point. The British believed that the Germans intended to take control of the French fleet. Based on this and a distrust of Hitler, the British Was Cabinet decided to take action.
In order to neutralize the French fleet, the Royal Navy began Operation Catapult. During this operation, the Royal Navy began an attack on the French fleet and destroyed most of it's ships, submarines and equipment.
As for casualties, over 1300 sailors and soldiers lost their lives during the Royal Navy invasion. The Germans might’ve not gain access to the French fleet, but at what cost?