The rulers of Japan and Germany, rather than Franklin Roosevelt, chose the moment at which the United States would enter the world war. Japan had decided back in early July to undertake the southward advance at the risk of war with the United States, the Japanese Navy had insisted on including an attack on the United States in its military plans, and Hitler had decided to declare war if Japan attacked. But Roosevelt obviously did not shrink from entry into the world war in early December 1941. His administration had adopted the objective of defeating all the Axis powers and had begun the military and the economic planning to achieve it. He had shared that objective publicly with the American people, a large majority of whom now accepted war as inevitable. In October, fully three-quarters of respondents to a Gallup poll said either that the United States would inevitably get into the war in Europe or that the United States was in the war already. Stark’s and Marshall’s last-minute memorandum suggested that the early months of the war might be perilous indeed, but the administration’s Victory Program could not possibly be implemented in peacetime. With the Germans now halted before Moscow, ultimate victory over the Axis seemed at least possible, and the time to enter the war had come.

From Monday, December 1, through Thursday, December 4, new Magic intercepts conveyed Tokyo’s instructions to its diplomatic representatives in London, Singapore, Manila, Hong Kong, Washington, and various Chinese cities to destroy their codes and other publications. On December 6 in Tokyo—December 5 in the United States—the Foreign Ministry told the Embassy in Washington to await the delivery of a long message giving the Japanese reply to Hull’s November 26 note. War was obviously imminent. We must now look at both the manner in which the Japanese had decided to begin it, and the reasons why the key commanders in the Far East disregarded their warnings and so much available evidence and remained almost completely unprepared on the morning of December 7.

Roosevelt’s November 25 statement that the Japanese “were notorious for making an attack without warning” was a simple historical fact. Against China in 1894, Russia in 1904, Manchuria in 1931, and China in 1937, the Japanese had struck without any preliminary announcement or declaration of war. In the first two cases, they had begun the war with at least partially successful attempts to destroy enemy fleets. Both the Japanese and U.S. navies had adopted the doctrines of Admiral Alfred Thayer Mahan, who believed that great battles between fleets decided wars....