Short answers, pulled from the story.
The attack on Mers-el-Kébir was a British naval bombardment on the 3rd of July 1940, targeting French warships at the naval base near Oran in French Algeria. It was the main part of Operation Catapult, a British plan to prevent the French fleet from falling into German or Italian hands after France signed armistice agreements with both Axis powers in June 1940.
The British bombardment killed 1,297 French servicemen at Mers-el-Kébir. The battleship Bretagne sank with 977 of her crew after the third British salvo detonated her magazine at 6:09 p.m. on the 3rd of July 1940.
Britain attacked the French fleet at Mers-el-Kébir to prevent French warships from falling into German or Italian hands after France's armistice with the Axis powers. The British War Cabinet feared that the French fleet, the second largest concentration of capital ships in Europe after the Royal Navy, could shift the naval balance of power if seized by Germany or Italy.
Admiral Marcel-Bruno Gensoul commanded the French warships at Mers-el-Kébir, including two older battleships, two newer Force de Raid battleships, and a flotilla of six destroyers. The British attacking force, known as Force H, was commanded by Admiral James Somerville and based at Gibraltar.
On the 19th of November 1942, German forces attempted to seize the French fleet at Toulon as part of Case Anton, the military occupation of Vichy France. French sailors scuttled every ship of military value before German troops could capture them, including the battleships Dunkerque and Strasbourg and seven modern cruisers, four heavy and three light.
Historians disagree. George Melton, writing in 2015, argued the attack was a tactical failure that allowed the modern battleship Strasbourg to escape and pushed dispersed French ships to concentrate at Toulon, where they were more vulnerable to Axis seizure. David Brown and Colin Smith, writing in 2004 and 2010 respectively, concluded that the demonstration of British resolve had succeeded and was received favourably by the governments of the United States, Turkey, Greece, and Brazil.