What Was World War II? Causes, Main Events and How It Ended
U.S. troops landing at Omaha Beach, Normandy, on 6 June 1944. Photo: Robert F. Sargent / U.S. Coast Guard. Public domain. Image record.
World War II was a global conflict in which the Allies defeated the Axis powers, led by Nazi Germany, Fascist Italy and Imperial Japan. Usually dated from 1939 to 1945, it combined wars of territorial conquest, resistance to occupation and the mobilization of entire societies. Fighting extended across Europe, Asia, Africa and the oceans, while civilians endured genocide, bombing, hunger and displacement.
The familiar dates describe the war beginning in Europe. In Asia, full-scale war between Japan and China had already begun in July 1937, following Japan’s occupation of Manchuria in 1931. These conflicts eventually became interconnected. The National WWII Museum explains that earlier expansion.
Why did the war start?
There was no single cause. The settlement after World War I left territorial disputes and resentment, but another world war was not inevitable. Hitler’s government chose rearmament and conquest; Nazi racial ideology made the seizure of territory in Eastern Europe a central objective. Italy and Japan also pursued imperial expansion. These ambitions mattered more directly than the idea that countries simply drifted into another conflict. USHMM documents these expansionist aims.
Britain and France initially tried to avoid war through concessions to Hitler, a policy known as appeasement. It failed to stop German expansion. Germany annexed Austria in 1938, gained the Sudetenland after the Munich agreement and occupied the Czech lands in March 1939. The National Army Museum outlines this escalation.
The immediate trigger in Europe was Germany’s invasion of Poland on September 1, 1939. Britain and France declared war two days later. The German-Soviet nonaggression pact had secretly divided parts of Eastern Europe into spheres of influence; Soviet forces invaded eastern Poland on September 17. The pact was therefore part of the immediate path to war, but it did not make the Soviet Union a member of the Axis. See the USHMM account of the pact.
Who fought on each side?
The main Axis powers were Germany, Italy and Japan. The Allies included Britain, China, the Soviet Union and the United States, alongside many other countries, governments in exile and resistance forces. Neither coalition remained unchanged throughout the war. Italy entered in 1940; its 1943 armistice was followed by German occupation and continued fighting within Italy. USHMM explains the Axis alliance and its changes.
The Soviet Union became a major Allied combatant after Germany invaded it in June 1941. The United States entered the war in December, following Japan’s attack on Pearl Harbor and subsequent German and Italian declarations of war. Cooperation joined governments with sharply different political systems. The U.S. Office of the Historian describes the wartime alliance.
A short chronology of the fighting
Major phases, not every campaign
1939–1940
Poland was divided. Germany overran much of Western Europe, including France. Britain continued fighting; the Luftwaffe failed to gain the air superiority needed for an invasion.
1941
Germany attacked the Soviet Union. Japan attacked Pearl Harbor and expanded its offensives across Asia and the Pacific. The war’s fronts became more closely connected.
1942–1943
Midway, El Alamein and Stalingrad marked major reversals for Axis forces in different theaters. Allied advances developed unevenly, rather than through one universal turning point.
1944–1945
The Normandy landings opened a major front in northwestern Europe as Soviet armies advanced from the east. In the Pacific, Allied forces moved toward Japan while war continued in China and Southeast Asia.
For the military sequence, see the RAF Museum on Britain’s defense, the Australian War Memorial on turning points and National Archives records of D-Day, June 6, 1944.
The Holocaust and the war against civilians
The Holocaust was the persecution and genocide of Europe’s Jews by Nazi Germany and its allies and collaborators. Six million Jews were murdered. War and occupation expanded the regime’s ability to carry out mass shootings, deportations and killing-center operations. Genocide was central to Nazi policy, not an accidental consequence of battlefield fighting. USHMM’s introduction explains the Holocaust.
Nazi persecution also targeted Roma, disabled people, Soviet prisoners of war and other groups, with different policies and death tolls. These victims should not be collapsed into an interchangeable category. USHMM distinguishes the groups and evidence.
Civilian suffering extended far beyond Europe: Japanese occupation involved massacres and coercion, while bombing and disrupted food supplies devastated communities across several continents. The National WWII Museum lists an estimated 45 million civilian deaths worldwide, while warning that estimates vary widely. No single exact total captures the losses reliably. Its casualty estimates show the scale and uncertainty.
When did World War II end?
In Europe: Germany’s surrender took effect on May 8, 1945. A further signing in Berlin and the time difference help explain the May 9 commemoration in Russia and some other countries. Germany’s defeat did not end the war against Japan. The National Archives preserves the surrender documents.
In Asia and the Pacific: the United States bombed Hiroshima on August 6 and Nagasaki on August 9. Soviet entry into the war against Japan added another major military shock. Japan announced its surrender in August, with Emperor Hirohito’s broadcast on August 15. The relative weight of the pressures behind the decision remains debated. The National WWII Museum places the ending in its Asian context.
The formal surrender was signed on September 2, 1945, aboard USS Missouri in Tokyo Bay. August marks the announcement; September marks the formal instrument. Read the document at the National Archives.
What changed after 1945?
The defeated powers faced occupation and reconstruction. The United States and Soviet Union emerged as dominant rivals, and disputes over Europe helped turn wartime cooperation into the Cold War. Peace between the former major belligerents did not mean the end of political repression or regional wars. The National Museum of American Diplomacy traces that transition.
The United Nations Charter entered into force on October 24, 1945, establishing a new framework for international cooperation. Meanwhile, independence movements challenged weakened colonial empires. Decolonization unfolded over decades, sometimes through negotiation and sometimes through war; it was not completed by Allied victory. UN Charter history; Office of the Historian on decolonization.
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