<P> On December 8, 1941, the United States Congress declared war (Public Law 77 - 328, 55 STAT 795) on the Empire of Japan in response to that country's surprise attack on Pearl Harbor the prior day . It was formulated an hour after the Infamy Speech of US President Franklin D. Roosevelt . Japan had sent a message to the United States to its embassy in Washington earlier, but because of problems at the embassy in decoding the very long message--the high - security level assigned to the declaration meant that only personnel with very high clearances could decode it, which slowed down the process--it was not delivered to the U.S. Secretary of State until after the Pearl Harbor attack . Following the U.S. declaration, Japan's allies, Germany and Italy, declared war on the United States, bringing the United States fully into World War II . </P> <P> The attack on Pearl Harbor took place before a declaration of war by Japan had been delivered to the United States . It was originally stipulated that the attack should not commence until thirty minutes after Japan had informed the US that it was withdrawing from further peace negotiations, but the attack began before the notice could be delivered . Tokyo transmitted the 5,000 - word notification--known as the "14 - Part Message"--in two blocks to the Japanese Embassy in Washington . However, because of the very secret nature of the message, it had to be decoded, translated and typed up by high embassy officials, who were unable to do these tasks in the available time . Hence, the ambassador did not deliver it until after the attack had begun . But even if it had been, the notification was worded so that it actually neither declared war nor severed diplomatic relations, so it was not a proper declaration of war as required by diplomatic traditions . </P>

When did the us declare war on japan in ww2