As World War II wound down, the Allied forces advanced on Berlin from two directions. The Americans, British and French resistance advanced from the west, and Russia from the east. Hitler committed suicide, the war ended and everyone headed home. Except not everyone did.
The Americans, British and French turned around and went home after helping prop up local governments of those who had opposed the Nazis. The Russians never really left. If they did leave, they propped up puppet regimes that the Soviet leaders in Moscow would exterminate if any resistance to Soviet rule cropped up. Instead of restoring democratic rule, the Soviets installed communist regimes.