The event that catalyzed the beginning of the United States' involvement in World War II was the attack on Pearl Harbor on December 7, 1941. This surprise military strike by the Japanese Imperial Navy targeted the U.S. naval base in Hawaii, resulting in significant losses of life and military assets. The attack united American public opinion against Japan, leading to the U.S. declaring war the following day. This marked a turning point in the war, as the United States became a major player in the Allied efforts to defeat the Axis powers.