Not correct, the Soviet Union invaded Poland on September 17, 1939; Germany, several days earlier, on September 1. By the middle of September 1939, Germany conquered about 40% of Polish territory, mostly the west side. Poland could not win that war, but at that time, still, meaningful forces were regrouped to the east side of the country. With the Soviet invasion, it would be suicidal to fight on two fronts; hence, Polish soldiers were ordered to stop the fight.