Stalin never gave his share back, keeping everything Hitler agreed to give him after reconquering the land. The reason Poland was pushed west and given German land after the war was so Stalin could keep his piece of pre-war Poland.
In 1999, my family and I visited a newly opened museum in Warsaw with some Polish college students. The museum was dedicated to Poland during WWII.
One of the students told of his aunt and uncle who had fled from the German invasion from the north, in Gdansk. His relatives took a train to Warsaw. As their train arrived at the station, Russian troops were arriving by train at the same station from the east. His relatives got back on a train to go home.
Thus, the Germans and the Russians both invaded Poland during the start of the war.
One of the stories of the museum was that Auschwitz, and other concentration camps, were first employed to imprison the Polish upper class. The Germans intended to destroy the upper class and leave the lower classes to use as slaves. This imprisonment of Polish people started in 1940. Using the concentration camps against the Jews started later, in 1942.
When I was in high school, I would check the bias of history textbooks by examining how they treated certain events. Many American history books neglected to mention that Stalin and Hitler were allies when the war broke out. Even after 30 years, the American academic community was covering for Joe Stalin and the Communists.
Stalin never gave his share back, keeping everything Hitler agreed to give him after reconquering the land. The reason Poland was pushed west and given German land after the war was so Stalin could keep his piece of pre-war Poland.
Instead, the Germans killed the elite at Katyn. Sad.
There was actually a two and a half week gap. The battle of Warsaw had begun before the Soviets invaded, although the city held out for another couple of weeks. The Russians didn't reach Warsaw until about 5 years later.