The Second World War was the global conflict fought primarily between the Allied powers and the Axis powers from 1939 to 1945. It began in Europe when Germany invaded Poland on 1 September 1939, prompting Britain and France to declare war on Germany on 3 September. The conflict expanded into a worldwide war involving Europe, North Africa, the Atlantic, the Soviet Union, East Asia, Southeast Asia and the Pacific.
Germany, Italy and Japan formed the principal Axis powers, while the principal Allied powers included Britain, France, the Soviet Union, China and the United States, although alliances and belligerents changed over time. The war involved unprecedented mobilisation of military forces and civilian economies, extensive aerial bombing, occupation, resistance, genocide and mass displacement. The Holocaust, the systematic murder of approximately six million Jews by Nazi Germany and its collaborators, was one of the central crimes of the conflict.
The origins of the Second World War involved several interacting developments rather than one isolated cause. The post First World War settlement left unresolved territorial and political disputes, while the global economic crisis of the 1930s contributed to political instability. Nazi Germany pursued territorial expansion and rejected major aspects of the Versailles settlement. Germany remilitarised the Rhineland in 1936, annexed Austria in 1938, obtained the Sudetenland through the Munich Agreement and occupied the Czech lands in March 1939.
Italy invaded Ethiopia in 1935 and Japan expanded through military conquest in East Asia, including the occupation of Manchuria in 1931 and the wider war against China from 1937. The failure of collective security and the inability of European powers to contain successive acts of aggression contributed to the international crisis.
The war in Europe developed rapidly during its opening phase. Germany invaded Poland using concentrated combinations of armour, motorised forces and air power, while the Soviet Union invaded eastern Poland under the terms of the German Soviet Non Aggression Pact and its secret protocol. In 1940 Germany invaded Denmark and Norway, followed by the Netherlands, Belgium, Luxembourg and France. France collapsed rapidly, and an armistice divided the country between German occupation and the collaborationist Vichy regime.
Britain continued the war after the fall of France, leading to the Battle of Britain and the Blitz. Germany subsequently invaded the Soviet Union in June 1941 under Operation Barbarossa, creating the largest land theatre of the war. The United States entered the war after the Japanese attack on Pearl Harbor on 7 December 1941.
The war was fought across multiple major theatres. In Europe, the Eastern Front became the largest and deadliest theatre, involving enormous armies and catastrophic civilian losses. In North Africa, British Commonwealth and Axis forces fought over Egypt, Libya and strategic supply routes before the Axis defeat in Tunisia in 1943. In the Atlantic, Allied and Axis naval forces fought a prolonged campaign concerning merchant shipping, submarines, convoy systems and control of sea communications.
In Asia and the Pacific, Japan conquered extensive territories during 1941 and 1942 before Allied counteroffensives gradually reversed Japanese expansion. China had already been fighting Japan since 1937. The global nature of the war meant that soldiers and resources came from Europe, North America, Africa, Asia, Australasia and the Middle East, while colonial populations were heavily involved in military and economic mobilisation.
Industrial production and logistics were central to the conflict. Aircraft, tanks, artillery, ships, submarines, trucks, ammunition and fuel required enormous industrial capacity. The United States became a major supplier of Allied equipment through programmes such as Lend Lease, while Britain mobilised its economy and empire extensively. The Soviet Union relocated substantial industrial capacity eastward after the German invasion and produced enormous quantities of tanks, artillery and aircraft.
Germany mobilised increasingly large portions of its economy but faced severe shortages of oil and other strategic resources. Japan also confronted resource constraints and sought territorial expansion partly to secure access to raw materials. The war therefore depended not only on battlefield tactics but also on industrial production, transport, energy supplies, scientific research and the ability to sustain long campaigns.
Civilian populations were central targets and participants in the war. Strategic bombing affected cities throughout Europe and Asia, while occupation regimes imposed requisitions, forced labour, deportations and political repression. Nazi Germany implemented the Holocaust and murdered Jews, Roma, disabled people, Soviet prisoners of war and other persecuted groups. The Holocaust developed through stages including exclusion, ghettoisation, mass shootings and systematic extermination in killing centres.
Japan committed numerous atrocities in occupied territories, including the Nanjing Massacre and widespread forced labour and sexual exploitation. The war also produced large scale civilian deaths through famine, siege, disease and displacement. The atomic bombings of Hiroshima and Nagasaki in August 1945 introduced nuclear weapons into warfare and remain subjects of extensive historical and ethical debate.
Allied strategy evolved through major conferences and military campaigns. Britain and the United States eventually pursued a strategy of defeating Germany while supporting the Soviet Union, before opening a major western European front through the Normandy landings on 6 June 1944. Soviet offensives meanwhile pushed German forces westward after major turning points including Stalingrad and Kursk.
In the Pacific, the United States and its allies used naval, air and amphibious operations to advance towards Japan, while China continued resistance on the Asian mainland. Germany surrendered unconditionally on 8 May 1945 after Soviet forces captured Berlin and Hitler died by suicide. Japan surrendered on 15 August 1945 following the atomic bombings, the Soviet declaration of war against Japan and the collapse of Japanese strategic prospects, with the formal surrender signed on 2 September 1945.
The consequences of the Second World War were global and long lasting. Tens of millions of people died, including soldiers and civilians, and vast areas of Europe and Asia suffered destruction. Germany was occupied and divided into occupation zones, while Japan was placed under Allied occupation. The United Nations was established in 1945, and international institutions including the International Monetary Fund and World Bank emerged from the postwar settlement.
The United States and Soviet Union became the principal superpowers, producing the Cold War. European colonial empires weakened substantially, while anti colonial movements gained strength across Asia, Africa and the Middle East. War crimes trials, particularly the Nuremberg Trials and Tokyo Trial, established important precedents concerning individual responsibility for international crimes. The Second World War therefore transformed international politics, borders, economies, military technology and the global balance of power.
