Invasion of Poland (1939)
In the first few days of the German invasion, Poland proved to be more difficult to invade than the Germans had expected. In the Battle of Mokra on September 2nd, the Poles repulsed an attack by a German Panzer division and forced its retreat. The German air force, unlike their invasions of other countries, did not easily overcome the outnumbered Polish Air Force. However, their great superiority in numbers made them gain air superiority after five to six days.