<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>
what started the war between us and japan
that country 's surprise attack on Pearl Harbor the prior day