World War II was the most devastating war in human history in terms of lives lost and material destruction. It began in 1939 as a European conflict between Germany and an Anglo-French coalition but eventually widened to include most of the nations of the world. It ended in 1945, leaving a new world order dominated by the United States and the USSR.

More than any previous war, World War II involved the commitment of nations' entire human and economic resources, the blurring of the distinction between combatant and noncombatant, and the expansion of the battlefield to include all of the enemy's territory. The most important determinants of its outcome were industrial capacity and personnel. In the last stages of the war, two radically new weapons were introduced: the long-range rocket and the atomic bomb. In the main, however, the war was fought with the same or improved weapons of the types used in World War I. The greatest advances were in aircraft and tanks.

The human cost of the war fell heaviest on the USSR, for which the official total, military and civilian, is given as more than 20 million killed. The Allied military and civilian losses were 44 million; those of the Axis, 11 million. The military deaths on both sides in Europe numbered 19 million and in the war against Japan, 6 million. The U.S., which had no significant civilian losses, sustained 292,131 battle deaths and 115,187 deaths from other causes. The highest numbers of deaths, military and civilian, were as follows: USSR more than 13 million military and 7 million civilian; China more than 3 million military and more than 10 million civilian; Germany 3.5 million and 3.8 million; Poland 120,000 and 5.3 million; Japan 1.7 million and 380,000; Yugoslavia 300,000 and 1.3 million; Romania 200,000 and 465,000; France 250,000 and 360,000; British Empire and Commonwealth 452,000 and 60,000; Italy 330,000 and 80,000; Hungary 120,000 and 280,000; and Czechoslovakia 10,000 and 330,000.

Perhaps the most significant casualty over the long term was the world balance of power. Britain, France, Germany, and Japan ceased to be great powers in the traditional military sense, leaving only two, the United States and the Soviet Union.