September 1 - 1939 - Nazi Germany Invades Poland
Officially, in school books and government records, World War II began on September 1, 1939, with the invasion of Poland by Germany. The Nazis had been slowly building up their military and annexing neighboring territories for years before they invaded Poland. Additionally, the Spanish Civil War was a proxy war among major powers and practice run for World War II. Yet more striking than the fact that conflicts which directly led to World War II happened well before September 1, 1939, after Poland was conquered and divided by the Nazis and the Soviet Union, little fighting happened in Western Europe. There, the war would not really begin until May 1940.