As the Allied forces moved in from the West to secure a German defeat in World War Two, the Soviet forces moved into Germany from the East.
While American and British forces concentrated on Western Europe, the Red army replaced Nazi forces in a number of countries in Eastern Europe.
The Americans and the British could do nothing about this while Nazi Germany remained undefeated.
After all, the USSR was an ally at this time.
However, it was clear that Stalin was very reluctant to give up control of Eastern Europe, which he saw as a Soviet sphere of influence.
President Roosevelt and Winston Churchill did not like the Soviet domination of Eastern Europe, but they needed the USSR as an ally and they could do very little to prevent Stalin’s military takeover of this region.