Berlin Wall at the Brandenburg Gate
, 9 November 1989. Note the graffiti Wie denn
("How now") over the sign warning the public that they are leaving West Berlin
The German reunification (German: Deutsche Wiedervereinigung) was the process in 1990 in which the German Democratic Republic (GDR (German: DDR)/East Germany) became part of the Federal Republic of Germany (FRG (German: BRD)/West Germany) to form the reunited nation of Germany, and when Berlin reunited into a single city, as provided by its then Grundgesetz constitution Article 23. The end of the unification process is officially referred to as German unity (German: Deutsche Einheit), celebrated on 3 October (German Unity Day) (German: Tag der deutschen Einheit). Following German reunification, Berlin was once again designated as the capital of united Germany.
The East German government started to falter in May 1989, when the removal of Hungary's border fence with Austria opened a hole in the Iron Curtain. It caused an exodus of thousands of East Germans fleeing to West Germany and Austria via Hungary. The Peaceful Revolution, a series of protests by East Germans, led to the GDR's first free elections on 18 March 1990, and to the negotiations between the GDR and FRG that culminated in a Unification Treaty. Other negotiations between the GDR and FRG and the four occupying powers produced the so-called "Two Plus Four Treaty" (Treaty on the Final Settlement with Respect to Germany) granting full sovereignty to a unified German state, whose two parts were previously bound by a number of limitations stemming from their post-World War II status as occupied regions.
The 1945 Potsdam Agreement had specified that a full peace treaty concluding World War II, including the exact delimitation of Germany's postwar boundaries, required to be "accepted by the Government of Germany when a government adequate for the purpose is established." The Federal Republic had always maintained that no such government could be said to have been established until East and West Germany had been united within a free democratic state; but in 1990 a range of opinions continued to be maintained over whether a unified West Germany, East Germany and Berlin could be said to represent "Germany as a whole" for this purpose. The key question was whether a Germany that remained bounded to the east by the Oder-Neisse Line could act as a "united Germany" in signing the peace treaty without qualification. Under the "Two Plus Four Treaty" both the Federal Republic and the GDR committed themselves and their unified continuation to the principle that their joint pre-1990 boundaries constituted the entire territory that could be claimed by a Government of Germany, and hence that there were no further lands outside those boundaries that were parts of Germany as a whole.
The united Germany is not a successor state, but an enlarged continuation of the Federal Republic. As such, the enlarged Federal Republic of Germany retained the West German seats in international organizations including the (later the European Union) and NATO, while relinquishing membership in the Warsaw Pact and other international organizations to which only East Germany belonged. It also maintains the United Nations membership of the old West Germany.