It is impossible to underestimate the importance of World War II (1939–45) in U.S. history. The country became involved in the conflict after the December 7, 1941, attack on Pearl Harbor in Hawaii by Japanese aircraft. From that moment, massive changes swept the nation, changes that would be felt for the rest of the century.

When the decade began, America continued to suffer from the affects of the economic depression (the Great Depression, 1929–41) that had lasted throughout the 1930s. Although the New Deal policies of President Franklin D. Roosevelt (1882–1945) had helped the nation avoid outright economic disaster, many Americans remained unemployed and business activity was stuck in a slowdown. Then, German leader Adolf Hitler (1889–1945) invaded Czechoslovakia and Poland in 1939. Americans looked at the war that was gripping Europe and felt lucky that they were not involved. The overriding attitude toward foreign wars was that America should...