The United States entered World War II on December 7, 1941, following the surprise attack on Pearl Harbor by the Imperial Japanese Navy. This attack led to significant loss of life and damage to the U.S. Pacific Fleet, prompting a shift in American public opinion and policy. The day after the attack, President Franklin D. Roosevelt delivered a speech to Congress, declaring it a "date which will live in infamy" and subsequently requested a declaration of war against Japan. This marked America's full-scale entry into the conflict, significantly altering the war's dynamics and leading to a united Allied effort against the Axis powers.