Sunday, 7 December 1941. Japanese carrier aircraft wreck the American battleships at Pearl Harbor and kill 2,400 people. They miss the carriers, which are at sea. Within hours Japan also strikes Malaya, Hong Kong and the Philippines. It turns an isolationist country into a factory with a grudge.
Why then
Japan has been at war in China for four years. When it moves into southern Indochina in July 1941, America freezes Japanese assets and cuts off its oil. The bet in Tokyo: smash the fleet, run wild for six months, and the Americans will bargain.
What they wanted
Admiral Yamamoto plans a crippling first blow he privately doubts can win a long war. Roosevelt gets the war he could not ask Congress for. Churchill gets the ally he has been praying for. Chiang, fighting Japan alone since 1937, gets allies at last.
What changed
On 11 December Hitler declares war on the United States, sparing Roosevelt the choice of which enemy to fight. Six months later, at Midway, American aircraft sink four of the carriers that struck Hawaii. The war is now planetary.
Still argued
Stories of Roosevelt 'letting it happen' do not survive the files. The live argument is how a government already reading Japan's diplomatic codes still missed the target.