I didn’t know about the agreement between Stalin and Hitler to invade Poland. Thanks for posting....
Another story that is buried is the agreement at the end of WW2 to force the repatriation of Russians who had move to Europe. They were forced to return to Russia. Then they were killed at the border or moved to Siberia to die.
...you are not alone....unless you are a history buff and have really read about the beginnings of what became the Second World War, you might not know this important fact. Stalin was a very smart and very evil man, but if I am not correct, he was totally surprised when Hitler launched his Operation Barbarossa, the invasion of the Soviet Union...
Read the book "The Rise And Fall of the Third Reich", by William L. Shirer. The twisted diplomacy in that age is remarkable.