World War II : Global military conflict that, in terms of lives lost and material destruction, was the most devastating war in human history. 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 World After World War I : Three major powers had been dissatisfied with the outcome of World War I. Germany, the principal defeated nation, bitterly resented the territorial losses and reparations payments imposed on it by the Treaty of Versailles. Italy, one of the victors, found its territorial gains far from enough either to offset the cost of the war or to satisfy its ambitions. Japan, also a victor, was unhappy about its failure to gain control of China.

Causes of the War : France, Great Britain, and the U.S. had attained their wartime objectives. They had reduced Germany to a military cipher and had reorganized Europe and the world as they saw fit. The French and the British frequently disagreed on policy in the postwar period, however, and were unsure of their ability to defend the peace settlement. The U.S., disillusioned by the Europeans' failure to repay their war debts, retreated into isolationism.

Cost of the War : World War II's basic statistics qualify it as by far the greatest war in history in terms of human and material resources expended. In all, 61 countries with 1.7 billion people, three-fourths of the world's population, took part. A total of 110 million persons were mobilized for military service, more than half of those by three countries: the USSR (22-30 million), Germany (17 million), and the United States (16 million). For the major participants the largest numbers on duty at any one time were as follows: USSR (12,500,000); U.S. (12,245,000); Germany (10,938,000); British Empire and Commonwealth (8,720,000); Japan (7,193,000); and China (5,000,000).

Most statistics on the war are only estimates. The war's vast and chaotic sweep made uniform record keeping impossible. Some governments lost control of the data, and some resorted to manipulating it for political reasons.

A rough consensus has been reached on the total cost of the war. In terms of money spent, it has been put at more than $1 trillion, which makes it more expensive than all other wars combined. The human cost, not including more than 5 million Jews killed in the Holocaust  who were indirect victims of the war, is estimated to have been 55 million dead—25 million of those military and 30 million civilian.

Economic Statistics : The U.S. spent the most money on the war, an estimated $341 billion, including $50 billion for lend-lease supplies, of which $31 billion went to Britain, $11 billion to the Soviet Union, $5 billion to China, and $3 billion to 35 other countries. Germany was next, with $272 billion; followed by the Soviet Union, $192 billion; and then Britain, $120 billion; Italy, $94 billion; and Japan, $56 billion.

Except for the U.S., however, and some of the less militarily active Allies, the money spent does not come close to being the war's true cost. The Soviet government has calculated that the USSR lost 30 percent of its national wealth, while Nazi exactions and looting were of incalculable amounts in the occupied countries. The full cost to Japan has been estimated at $562 billion. In Germany, bombing and shelling had produced 4 billion cu m (5 billion cu yd) of rubble.

Human Losses : 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 (it is including the atomic bombing over Hiroshima on August 6, the other over Nagasaki on August 9. and the  number killed in Hiroshima at 66,000 to 78,000 and in Nagasaki at 39,000. Japanese estimates gave a combined total of 240,000.) 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 13,000,000 military and 7,000,000 civilian; China 3,500,000 and 10,000,000; Germany 3,500,000 and 3,800,000; Poland 120,000 and 5,300,000; Japan 1,700,000 and 380,000; Yugoslavia 300,000 and 1,300,000; 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.