The major event that took place on December 7, 1941, in U.S. history was the attack on Pearl Harbor by the Japanese Imperial Navy. This surprise military strike targeted the U.S. naval base in Hawaii, aiming to incapacitate the Pacific Fleet. The attack resulted in significant loss of life and damage to ships and aircraft. It led to the United States' formal entry into World War II, as President Franklin D. Roosevelt declared war on Japan the following day. Pearl Harbor has since become a symbol of American resilience and the catalyst for wartime unity.