The attack on Pearl Harbor on December 7, 1941, had a significant impact on U.S. involvement in World War II. This surprise military strike by the Japanese Navy led to the United States formally entering the war the following day, when Congress declared war on Japan. The attack galvanized American public opinion, which had been largely isolationist, and united the nation in support of the war effort. The U.S. mobilization for war dramatically increased military recruitment, production, and economic activity, ultimately leading to a major shift in the conflict and contributing to the Allied victory in both the European and Pacific theaters.