1989–1991 unification process of Germany
Source (Wikipedia): German reunification

German reunification was the process of re-establishing Germany as a single full sovereign state, which took place between 9 November 1989 and 15 March 1991. The “Unification Treaty” entered into force on 3 October 1990, dissolving the German Democratic Republic and integrating its recently re-established constituent federated states into the Federal Republic of Germany to form present-day Germany. This date has been chosen as the customary German Unity Day, and has thereafter been celebrated each year as a national holiday in Germany since 1991. As part of the reunification, East and West Berlin were also de facto united into a single city, which eventually became the capital of Germany.