On this day in history, September 1, 1939, World War II began. The war started when Germany invaded Poland. The invasion was a result of the signing of the Nazi-Soviet Non-Aggression Pact between Germany and Russia on August 23, 1939. In a secret clause of the agreement, the ideological enemies agreed to divide Poland between them.
Hitler gave orders for the Poland invasion to begin on August 26, but on August 25 he delayed the attack when he learned that Britain had signed a new treaty with Poland, promising military support should it be attacked. When Germany attacked Poland, Britain and France declared war on Germany.