1940s: World War II rages all over Europe as England, France and the Allied Powers fight Hitler's Nazi regime. When Japan bombs Pearl Harbor on December 7, 1941, the United States enters the war on the side of the Allies.

Today: Although the decade is free of largescale war, several regional conflicts pose threats to world peace. Problems in the Middle East, in Africa, and in the Balkans force the United Nations to send troops around the world.

1940s: In Argentina, Juan Perón is elected to the presidency, but quickly becomes a dictator with the support of the military.

Today: After decades of repression—particularly of the press and intellectuals—Argentina moves toward a more open government with the return of a civilian government in the 1980s. The government puts forth a concerted effort to find the bodies of people who "disappeared...