Explanation: During the start of WW2, Poland was invaded by Nazi Germany. Poland had a war plan for this that involved buying time for Britain and France to come to their aid. Unfortunately, the Soviet Union invaded from the east once Poland's forces were committed against Germany in the west, leading to the total collapse of the Polish defense. The Soviets then met and shook hands with the Nazis over their fantastic cooperation in pursuit of the partition and genocide of Poland.
The Soviets then spent the next year-and-a-half imprisoning 500,000 Poles (approximately 10% of the Polish population under their control) and murdering 150,000 (approximately 3% of the total Polish population under their control); having already murdered 100,000 Poles in Soviet territory the year before that
They were interrupted after that by the Nazis invading the Soviet Union itself.