What caused World War II?

The war in Europe began on September 1, 1939, when Germany invaded Poland, prompting Britain and France to declare war two days later. The path there ran through the previous two decades. The Treaty of Versailles imposed territorial losses, disarmament, and heavy reparations on Germany, creating grievances that nationalist movements exploited. The Great Depression devastated the German economy and radicalized politics across Europe. After 1933 the Nazi regime systematically violated the treaty, remilitarizing the Rhineland in 1936, annexing Austria in 1938, and taking Czechoslovakia in stages after the Munich Agreement, in which Britain and France conceded territory in exchange for promises that were immediately broken.

Several other threads converged. Japan had been expanding in Asia since invading Manchuria in 1931 and launched full-scale war against China in 1937, so a major conflict was already underway in the Pacific before Europe ignited. Italy under Mussolini invaded Ethiopia in 1935 with minimal international consequence. The League of Nations proved unable to enforce anything against determined aggressors, and the Nazi-Soviet Pact of August 1939 removed Germany's fear of a two-front war and secretly divided Eastern Europe, clearing the way for the invasion of Poland.

The United States entered after Japan attacked Pearl Harbor on December 7, 1941, and Germany declared war on the United States days later. The scale that followed is difficult to convey: an estimated 70 to 85 million deaths, the majority civilians, including roughly six million Jews murdered in the Holocaust along with millions of Roma, disabled people, Soviet prisoners, and others targeted by the Nazi state. The war ended in Europe in May 1945 and in the Pacific in September after the atomic bombings of Hiroshima and Nagasaki. Its settlement produced the United Nations, the division of Europe that defined the Cold War, decolonization across Asia and Africa, and the international legal framework established at Nuremberg.