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.
Later Russia seized Poland and attacked Finland and the Baltic States. Germany conquered Denmark and Norway and defeated France. Germany then won Poland back from Russia and the pact between Germany and Russia ended when Germany attempted to invade Russia.
The waves of invasion continued when German and Italian forces won the Balkans. Invasions of North Africa, Normandy and Italy brought US, British, French and other allied troops to Germany to retaliate. Germany surrendered by May 1945. Most of the world broke out in war between 1939 and 1945.