Adolph Hitler started World War II on Sept. 1, 1939, by invading Poland. While he had earlier taken over Austria and Czechoslovakia, these actions were ignored or explained away by European leaders who did not want a war.
Italy, Japan and Germany had all made earlier moves presaging World War II. In 1931, Japan took over Manchuria, and in 1935, Italy took over Ethiopia. Hitler's first warlike move was transporting troops and weaponry into the Rhineland in 1936, contrary to the World War I-ending Treaty of Versailles. However, none of these aggressions seemed like more than a localized conflict until Hitler invaded Poland, forcing Britain and France to declare war on Germany to protect an ally. Subsequent alliances and conflicts drew in countries from every continent.