Poland's DefeatOn September 1, 1939, German troops invaded Poland. Polish defenses crumbled before the German onslaught of tanks, motorized vehicles and attacks by dive-bombers on the civilian population. The German theory of Blitzkrieg ("lightning war") involved massive concentrated attack.
After two weeks Germany controlled western Poland except for Warsaw, which held out for two more weeks. Meanwhile, on September 17, 1939, the Soviet Union invaded Poland from the east according to the German-Soviet Non-Aggression Pact signed in August 1939, which divided Poland into spheres of interest for each country.
Source: Encyclopedia of the Holocaust.