In the first few days of the German invasion, the Poles proved to be more difficult than the Germans expected. In the Battle of Mokra on September 2nd, the Poles repulsed an attack by a German Panzer division forcing their retreat. The German air force, unlike their invasions of other countries, did not easily overcome the outnumbered Polish Air Force. However, due to their great superiority in numbers, they gained air superiority after 5 to 6 days.