<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 for 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, but that was not the intent of the Japanese leadership . It was originally stipulated that the attack should not commence until thirty minutes after Japan had informed the United States that it was withdrawing from further peace negotiations . It was the intent of the Japanese to uphold the conventions of war while still achieving surprise, 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, decoding and translation took too long for the ambassador to deliver it in time . Even so, the notification was worded so that it actually neither declared war nor severed diplomatic relations . </P>

When did the usa declare war on japan