The United States' full involvement in World War II was a pivotal moment in the history of the 20th century. The causes of U.S. involvement included the Japanese attack on Pearl Harbor in 1941, which brought the U.S. into the war in the Pacific, and Germany's declaration of war on the U.S. a few days later. The U.S. rapidly mobilized its economy and military, sending troops and equipment to both the Pacific and European theaters of war. American forces fought alongside their allies in the decisive battles of the war, including the D-Day invasion of Normandy and the Pacific island-hopping campaign. The war had a profound impact on the U.S., transforming the country into a global superpower and setting the stage for the Cold War. The war also led to significant social and economic changes, including the expansion of women's roles in the workforce and the growth of the federal government's role in the economy.