The role of the United States in ending World War II was pivotal, particularly in the European and Pacific theaters. After officially entering the war in 1941 following the attack on Pearl Harbor, the U.S. mobilized its vast resources to support Allied forces through military engagement, supply shipments, and strategic bombing campaigns. In Europe, American forces played significant roles in key victories, such as the D-Day invasion in 1944, which contributed to the liberation of Western Europe from Nazi control. In the Pacific, the U.S. conducted island-hopping campaigns and ultimately dropped atomic bombs on Hiroshima and Nagasaki, leading to Japan's surrender in 1945.