How did East Germany become part of the Federal Republic of Germany in 1990?

The reunification of Germany in 1990 was the result of a rapid political collapse of the German Democratic Republic (GDR) and subsequent diplomatic negotiations.

The process was triggered by mass protests and a surge of refugees fleeing to the West through newly opened borders in Eastern Europe. Following the accidental opening of the Berlin Wall on November 9, 1989, the GDR government lost its authority. In March 1990, East Germany held its first free, multiparty elections, where parties favoring speedy reunification won a decisive victory.12

To achieve unity, the two German states had to secure the approval of the four World War II Allies. Through the "two plus four" negotiations, the Federal Republic of Germany and the GDR, along with the United States, the United Kingdom, France, and the Soviet Union, settled issues regarding security and sovereignty. A unification treaty was ratified by the Bundestag and the East German People’s Chamber, officially taking effect on October 3, 1990. The GDR joined the Federal Republic as five new Länder, and a unified Berlin became a single Land.13