In what year did World War II officially begin?

World War II officially began in 1939, generally dated to September 1, when Nazi Germany invaded Poland. Britain and France declared war on Germany two days later, turning the invasion into a wider European conflict.

Germany’s attack followed years of unresolved tensions after World War I, the rise of fascist governments, German territorial expansion, and the failure of diplomatic efforts to contain Adolf Hitler. The Molotov–Ribbentrop Pact with the Soviet Union also helped clear the way for the invasion by secretly dividing parts of Eastern Europe into spheres of influence.

The year 1939 is the standard answer for the war’s beginning, but historians sometimes discuss earlier starting points. Japan had invaded Manchuria in 1931, and the Second Sino-Japanese War began in 1937. Those conflicts later became part of the global war, but most general histories use the German invasion of Poland as the conventional starting date.

The war ended in 1945 after Germany surrendered in May and Japan formally surrendered in September.