On this date in 1940, the Royal Navy took the French battle fleet under fire while it was anchored in its base at Mers-el-Kébir in French Algeria.
This was biggest and best-known part of Operation Catapult, the British attempt to seize or disable French warships to keep them from falling into the hands of the Nazi war machine after the Armistice between the French and their German invaders was signed June 22nd.
Back in Britain, French ships in the harbors at Plymouth and Portsmouth were boarded and taken under British control, sometimes after armed resistance. The commander of the French flotilla in Alexandria, Egypt agreed to disarm his ships and stay in port. Only at Mers-el-Kébir did negotiations fall apart, resulting in the Hood, Valiant, and Resolution shelling the bejeezus out of the anchored French battleships, along with torpedo and mine attacks from aircraft flying from HMS Ark Royal.
Some 1,300 French sailors and officers were killed, a thousand of them on the pre-WWI dreadnought Bretagne, which suffered a magazine explosion after multiple direct hits from the Brits' 15" guns, rolled over and sank.
This did nothing good for Anglo-French relations, as you may imagine.
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