The combined air-and-sea attack was conducted by the Royal Navy after the Second Armistice at Compiègne between Germany and France on 22 June. The only continental ally of Britain had been replaced by a government administered from Vichy, which inherited the French navy (Marine nationale). Of particular significance to the British were the seven battleships of the Bretagne, Dunkerque and Richelieu classes, the second largest force of capital ships in Europe after the Royal Navy. The British War Cabinet feared already that France would hand the ships to the Kriegsmarine, giving the Axis an advantage in the Battle of the Atlantic. Admiral François Darlan, commander of the French Navy, promised the British that the fleet would remain under French control but Winston Churchill and the War Cabinet judged that the fleet was too powerful to risk an Axis take-over.
After the attack at Mers-el-Kébir and the Battle of Dakar, French aircraft raided Gibraltar and the Vichy government severed diplomatic relations with the United Kingdom. The attack created much rancour between France and Britain but also demonstrated to the world that Britain intended to fight on. The attack is controversial and the motives of the British are debated. In 1979, P. M. H. Bell wrote that "The times were desperate; invasion seemed imminent; and the British government simply could not afford to risk the Germans seizing control of the French fleet... The predominant British motive was thus dire necessity and self-preservation".
The French thought they were acting honourably in terms of their armistice with Nazi Germany and were convinced they would never turn over their fleet to Germany. Vichy France was created on 10 July 1940, one week after the attack and was seen by the British as a puppet state of the Nazi regime. French grievances festered for years over what they considered a betrayal by their ally. On 27 November 1942, the scuttling of the French fleet in Toulon foiled Operation Anton, a German attempt to capture the fleet.
After the Fall of France in 1940 and the armistice between France and Nazi Germany, the British War Cabinet was apprehensive about the Germans acquiring control of the French navy from the government of Vichy France. The French and German navies combined could alter the balance of power at sea, threatening British imports over the Atlantic and communications with the rest of the British Empire. That the armistice terms at article eight paragraph two stated that the German government "solemnly and firmly declared that it had no intention of making demands regarding the French fleet during the peace negotiations" and that similar terms existed in the armistice with Italy, were considered to be no guarantee of the neutralisation of the French fleet. On 24 June, Darlan assured Winston Churchill against such a possibility. Churchill ordered that a demand be made that the French Navy (Marine nationale) should either join with the Royal Navy or be neutralised in a manner guaranteed to prevent the ships falling into Axis hands.
At Italian suggestion, the armistice terms were amended to permit the French fleet to stay temporarily in North African ports, where they might be seized by Italian troops from Libya. The British made contingency plans to eliminate the French fleet (Operation Catapult) in mid-June, when it was clear that Philippe Pétain was forming a government with a view to signing an armistice and it seemed likely that the French fleet might be seized by the Germans. In a speech to Parliament, Churchill repeated that the Armistice of 22 June 1940 was a betrayal of the Allied agreement not to make a separate peace. Churchill said "What is the value of that? Ask half a dozen countries; what is the value of such a solemn assurance? ... Finally, the armistice could be voided at any time on any pretext of non-observance ..."