The significant event that took place on December 7, 1941, leading the U.S. to enter World War II was the attack on Pearl Harbor. This surprise military strike was conducted by the Imperial Japanese Navy Air Service against the United States naval base in Hawaii. The attack resulted in extensive destruction of the Pacific Fleet, with significant loss of life and damage to ships and aircraft. In response to this aggressive act, the United States declared war on Japan the following day, marking a decisive shift in U.S. foreign policy from isolationism to active involvement in the global conflict.