The major event that started on December 7, 1941, in the United States was the surprise military 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. In response, the United States formally entered World War II, declaring war on Japan the following day. Pearl Harbor marked a turning point in U.S. military involvement and catalyzed a unified national effort against the Axis powers, eventually leading to victory in both the Pacific and European theaters of the war.