The great and all powerful Soviet Union, and then after 1949 the communist german party took controlled which was maintained by the United Soviet Socialist Republic (USSR). Also I should mention that the Allies wanted to split the city of Berlin up since it was fair.. So Berlin was split in 4 parts and thats why you hear the name "Berlin Wall" which was the Soviet Wall surrounding their side of the city..
This image is from the battle of Berlin won by the Soviet Union in 1945
This image is from the battle of Berlin won by the Soviet Union in 1945