The 1940s was the decade from 1940 to 1949, a period dominated by World War II and its aftermath. For the first half of the decade, much of the world was at war. The United States entered the conflict in 1941 after the attack on Pearl Harbor, and American factories shifted to producing tanks, planes, and ships instead of cars and refrigerators. Millions of Americans served in the military overseas, while families at home rationed food, gasoline, and other supplies to support the war effort.

The war ended in 1945, first in Europe in May and then in the Pacific in August after the United States dropped atomic bombs on Japan. The second half of the decade focused on rebuilding and recovery. Europe and Japan worked to reconstruct destroyed cities, while the United States experienced an economic boom. New technologies developed during the war, like radar and jet engines, began finding peacetime uses.

In American culture, the 1940s brought big band music, classic Hollywood films, and the beginning of suburbanization as families moved from cities to newly built neighborhoods. Jackie Robinson broke baseball's color barrier in 1947, becoming the first Black player in the major leagues in the modern era. The decade also saw the beginning of the Cold War between the United States and the Soviet Union, a tense rivalry that would shape global politics for the next forty years.