World War II was the most widespread and deadly war in history. It was a total war, raining as much destruction on civilians as on armed forces. It involved brutalities on a scale never before known. One of these was the attempt by Germany to exterminate the entire Jewish population of Europe.

Unlike World War I, World War II was fast-moving and mobile. There were periods of stalemate, especially in Italy, but none that compared with the lengthy periods of stalled trench warfare in World War I.

The tank and the airplane had been introduced during World War I, but their use was more widespread and effective in World War II. The aircraft carrier emerged as the most important surface warship, and the submarine caused great damage, as it had in the earlier conflict. New military developments of World War II included the use of parachute troops, radar, sonar, and suicide airplanes. The most destructive weapon ever developed—the atomic bomb—was used in the closing days of the war.

Women played a far greater part in World War II than in any previous conflict. They served in the armed forces as nurses, technicians, and clerks, and also took the place of men in factories and on farms. Some fought alongside men as guerrillas.

Among the results of World War II was a greatly changed map of the world. The Soviet Union annexed territory along its western borders, and the map was redrawn throughout the rest of Europe. Germany and Korea were each divided into Communist and non-Communist sectors. All of eastern Europe and most of the Balkans came under Soviet domination. The European colonies in Africa and Asia began demanding independence. A Jewish state—Israel—was carved out of Palestine.

The most profound political effects after World War II were the emergence of the United States and the Soviet Union as the two leading world powers (often referred to as the superpowers), the rise of Communist China, and the division of the world's nations into three groups—those allied with the United States, those allied with the Soviet Union, and those not committed to either.