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.

Answer this question: when did the united states declared war on japan? Let's give stream of consciousness first:
The important part for answering this question is: 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.
So, the final answer is December 8 , 1941.