Late one stormy night at 10 Downing Street, Winston Churchill summoned Max Beaverbrook, A. V. Alexander, and Sir Dudley Pound to decide what to do about the French fleet. British leaders feared that if Germany gained control of those warships, the naval balance, especially in the Mediterranean, would shift dangerously against Britain; intelligence that Germany possessed French naval codes deepened that fear. Although Churchill called the choice hateful and unnatural, Beaverbrook argued that Hitler would inevitably coerce France into surrendering the fleet. Churchill accepted that logic, gave the order to proceed with Operation Catapult, and then, overwhelmed by the human cost of attacking former allies, wept in the Downing Street garden.

Vice Admiral Sir J. F. Somerville, commanding Force H from Gibraltar, received final orders to confront the French squadron at Mers el-Kébir. Somerville was instructed first to offer Admiral Marcel Gensoul several options: join Britain in continuing the war, sail to a British port, or go to the West Indies to disarm or transfer the ships for safekeeping; otherwise, the British would use force. As Force H sailed, the chapter shifts briefly to Berlin, where Adolf Hitler, frustrated that Churchill would not negotiate, asked his military chiefs to examine the feasibility of invading Britain. Even then, Hitler treated invasion cautiously and stressed that Germany would need complete air superiority over the RAF before attempting it.

On July 3, British officers tried to open negotiations off Oran, but Gensoul at first refused to meet them and then delayed while his ships prepared to sail. British reconnaissance reported steam up, manned bridges, and increasing readiness in the harbor, so Somerville ordered aircraft from Ark Royal to lay mines at the harbor mouth. A face-to-face meeting finally began aboard the French flagship Dunkerque, but it accomplished nothing, and London concluded that both Gensoul and, by hesitation, Somerville were letting time slip away. Meanwhile, in British ports such as Portsmouth and Plymouth, the seizure of accessible French ships went mostly smoothly, though the submarine Surcouf resisted and a brief fight left one French sailor and three British sailors dead.

With night approaching and the talks fruitless, the Admiralty ordered Somerville to settle the matter quickly. At 5:54 p.m., after the deadline had passed, Force H opened fire from long range. The French battleship Bretagne exploded and sank, other ships were badly hit, and French guns answered back, but after about ten minutes Somerville ceased fire once the harbor batteries fell silent. The action killed 1,297 French officers and sailors, while Force H suffered no casualties. Churchill, receiving the news in London, was deeply shaken and kept repeating that it was terrible, yet he also believed the attack had achieved a crucial strategic and political effect by proving that Britain would fight on rather than seek terms.

The next day, Churchill presented the action to the House of Commons as a melancholy necessity and won an overwhelming response from Parliament, the public, and much of Britain, even as some Admiralty officers denounced the attack as treachery and French naval officers bitterly reproached Somerville. Churchill's speech turned grim news into a demonstration of resolve, reinforcing his standing at home and abroad. The emotional aftershock lingered socially as well: at a Downing Street lunch, Charles de Gaulle remarked that the French fleet might prefer to turn its guns on the British, prompting Clementine Churchill to rebuke him sharply in French. Churchill tried to smooth over the scene with a joke, but Clementine stood by her words, and de Gaulle later sent her flowers in apology.