World War II was a global armed conflict lasting from 1 September 1939 to 2 September 1945, involving most of the world's nations organized into two opposing military alliances: the Allies and the Axis powers. It remains the deadliest conflict in recorded human history, with estimates of total deaths ranging from 70 to 85 million, the majority of whom were civilians. The war reshaped the political, territorial, and social order of virtually every inhabited continent and gave rise to the postwar international institutions and power structures that defined the latter half of the twentieth century.

The conflict encompassed multiple theaters of operation, including Europe, North Africa, East Asia, the Pacific, and portions of the Middle East and South Asia. Principal Axis powers were Nazi Germany, the Kingdom of Italy, and the Empire of Japan, along with several allied and satellite states. The major Allied powers included the United Kingdom and its Commonwealth nations, the Soviet Union, the United States, the Republic of China, and France (initially through its government-in-exile and the Free French Forces). Dozens of other nations participated in varying capacities.

The war in Europe originated in the expansionist foreign policy of Adolf Hitler's National Socialist government in Germany and the failure of a collective European response to halt that expansion. The Pacific war had parallel origins in Imperial Japan's expansionist campaigns in China, beginning with the Second Sino-Japanese War (1937) and its subsequent advance into Southeast Asia and the Pacific. These two theaters were connected diplomatically through the Tripartite Pact (1940) but operated largely independently in military terms.

The conflict is notable for the Holocaust - the systematic, state-sponsored murder of approximately six million Jews, along with millions of others (including Romani people, persons with disabilities, political prisoners, and others) by the Nazi German regime and its collaborators. It also involved the use of nuclear weapons for the first time in warfare, with the United States dropping atomic bombs on Hiroshima (6 August 1945) and Nagasaki (9 August 1945). The war's conclusion produced an altered global order characterized by the emergence of the United States and Soviet Union as superpowers, the beginning of the Cold War, the establishment of the United Nations, and the accelerated dissolution of European colonial empires.

The origins of the war are traced variously to the unresolved tensions of World War I, the punitive terms of the Treaty of Versailles (1919), the global economic collapse of the 1930s, the rise of fascist and ultranationalist governments in Germany, Italy, Japan, and elsewhere, and the failure of collective security mechanisms in the interwar period. A full account of the war's causes, course, and consequences is provided at World War II - History.

There is broad scholarly consensus on the war's major chronological facts, the character and extent of the Holocaust, casualty ranges, and the principal military and diplomatic turning points. These are addressed at World War II - Historical Consensus. Questions of moral responsibility, the strategic necessity of particular decisions (such as the atomic bombings or the Allied strategic bombing campaign), and the war's relationship to colonialism and race remain subjects of active scholarly debate.

On the atomic bombings of Japan: A long-running debate concerns whether the use of atomic weapons against Hiroshima and Nagasaki was morally justified and militarily necessary. Some historians argue it shortened the war and prevented greater casualties from a ground invasion; others contend Japan was near surrender and the bombings were unnecessary or served postwar geopolitical objectives rather than purely military ones. See Atomic Bombings - Justified and Atomic Bombings - Unjustified.

On Allied strategic bombing: The systematic bombing of German and Japanese cities, which killed hundreds of thousands of civilians, is debated as a question of both military effectiveness and moral permissibility under the laws of war. See Strategic Bombing - Debate-viewpoint.

On Soviet responsibility and the Eastern Front: The scale of the Eastern Front - where the majority of the war's land combat occurred - and the role of the Soviet Union as both aggressor (under the Molotov-Ribbentrop Pact, 1939-1941) and principal Allied land power raise contested questions about how Soviet conduct should be evaluated morally and historically. See Soviet Role in World War II.

On collaboration and resistance: The extent to which occupied populations collaborated with Axis occupiers versus resisted them varies significantly by country and has been the subject of substantial national historiographical debate, particularly in France, Poland, the Netherlands, and Ukraine. See Collaboration and Resistance - Debate-viewpoint.

On colonialism and the war: The relationship between the war and European colonialism is a matter of ongoing scholarly and political discussion. Some historians argue that the war's rhetoric of freedom was applied selectively, that colonial subjects were mobilized under coercive conditions, and that the Allied cause offered little to peoples living under foreign rule. Others argue that the Allied cause represented a genuine defense against totalitarianism that benefited colonized peoples in the long run. See World War II and Colonialism - Debate-viewpoint and Colonial and Native Relations - History.

On German collective guilt: The degree to which ordinary German citizens bore moral responsibility for Nazi crimes - as distinct from the regime's leadership - has been debated by historians and philosophers since the war's end. See German Collective Guilt - Debate-viewpoint.

• World War II - History
• World War II - Historical Consensus
• World War II - Debate
• World War I
• The Holocaust
• The Cold War
• Colonial and Native Relations - History
• Nuclear Weapons - Ethics Debate
• Strategic Bombing - Debate

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