The event that prompted the U.S. to enter World War II was the Japanese attack on Pearl Harbor on December 7, 1941. This surprise military strike targeted the U.S. naval base in Hawaii, resulting in significant damage to the Pacific Fleet and the loss of thousands of lives. In the wake of the assault, President Franklin D. Roosevelt addressed Congress, famously referring to the date as "a day that will live in infamy." The attack galvanized public opinion in favor of entering the war, leading to a formal declaration of war against Japan on December 8, 1941, and marking the U.S.'s active involvement in World War II.