The eastern area of Germany that was held by the Soviets became the so-called German Democratic Republic (RDA), and the other three formed a single area, which was the Federal Republic of Germany (FRG).
In the following years, the western zone began to prosper while the Soviet side suffered economic problems. This, caused millions of Berliners who lived under the communist regime to migrate to West Berlin.
It is for this reason that the authorities of the GDR decided to build a provisional wall that divided the city in two.