The Nazi's invaded Poland on September 1, 1939. Hitler and the Nazi's wanted to expand for "living space" for the German people. Hitler started the German expansion in 1938 with the union with Austria and then the invasion of Sudentenland and Czechoslovakia.
3/12/38: Germany and Austria form Union
10/15/38: Germany occupies Sudentenland
3/15/39: Nazi's take over Czechoslovakia
5/22/39: Nazi's sign "Pact of Steel" with Italy
8/23/39: Nazi's sign a pact with the Soviets
9/1/39: Nazi's invade Poland