World War

The prosperity of the decade that followed World War I came to a sudden end in 1929 when the stock market crashed and the Great Depression began. It ushered in an era of increased federal involvement in economic and social policy under President Franklin D. Roosevelt. His New Deal legislation revolutionized the country, but full economic recovery was still not achieved until war production became massive on the eve of World War II. The Japanese attack on Pearl Harbor brought the United States into World War II on the side of Britain and the Soviet Union against the fascist nations of Germany, Japan, and Italy. The war effort galvanized the American economy's productive capacity, and after victory was achieved in 1945 the United States experienced three decades of unprecedented economic growth and prosperity.
The Allied victory in 1945 left the United States the leader of the Western world, deeply involved in the reconstruction of Europe and Japan, but embroiled in 40-year-long rivalry with the Soviet Union that became known as the Cold War. In 1949 the United States formed the North Atlantic Treaty Organization in an effort to counter the Soviet military presence in eastern Europe, and a Soviet-inspired attack on South Korea involved the United States in the Korean War (1950-53), which ended in stalemate. The United States subsequently became involved in the Vietnam War (1955-75) in an effort to prevent communist North Vietnam from taking over South Vietnam. The prolonged and unsuccessful American war effort ended in a withdrawal of the United States from the conflict in 1973 and the fall of South Vietnam to the communists two years later.