After 17 days at war against the Nazi Germans in the west, Poland was invaded from the east by the Soviet Union, in fulfillment of the secret Molotov–Ribbentrop Pact, a secret pact between the Nazis and the Communists whose goal was to erase Poland and the Polish people from the map and from humanity. Didn’t work out so well in the long run…..
It’s a long and complicated story, and not a pretty one. You can read more about it here: http://en.wikipedia.org/wiki/Soviet_invasion_of_Poland. As Sir Wintson Churchill said, “Those who fail to learn from history are doomed to repeat it.” Let’s not repeat it, please….