The United States formally entered World War II on December 8, 1941, following the Japanese attack on Pearl Harbor the previous day. This surprise military strike led to the United States declaring war on Japan, marking its entry into the conflict that had already engulfed Europe and Asia. The attack galvanized American public opinion, which had largely favored isolationism prior to this event. Once involved, the U.S. mobilized its resources to support the Allied powers, contributing significantly to military efforts against the Axis forces, while also leading to a profound transformation in the American economy and society during and after the war.