In what year did World War II begin in Europe?

World War II began in Europe in 1939. The conventional starting point is 1 September 1939, when Nazi Germany invaded Poland under Adolf Hitler. Britain and France declared war on Germany two days later, turning the German-Polish conflict into a wider European war.

The invasion followed years of tension created by the Treaty of Versailles, German rearmament, fascist expansion, and the policy of appeasement. Germany had already annexed Austria and taken the Sudetenland, but the attack on Poland crossed the decisive line for Britain and France.

The Soviet Union invaded eastern Poland on 17 September 1939 under its secret agreement with Germany. The campaign ended with Poland divided between the two powers. Japan had already invaded Manchuria in 1931 and was fighting China from 1937, so some historians discuss an earlier Asian starting point for the wider global conflict. For the European war, however, 1939 is the accepted answer.