from Wikipedia:
https://en.wikipedia.org/wiki/German_reunification
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 Democratic Republic 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.
and
Germany was officially reunited at 00:00 CEST on 3 October 1990. East Germany joined the Federal Republic as the five
Länder (states) of
Brandenburg,
Mecklenburg-Vorpommern,
Saxony,
Saxony-Anhalt and
Thuringia. These states were the five original states of East Germany, but were abolished in 1952 in favour of a centralised system. As part of the 18 May treaty, the five East German states were reconstituted on 23 August. At the same time,
East and
West Berlin reunited into one city, which became a city-state along the lines of the existing city-states of
Bremen and
Hamburg.
Berlin was still formally under Allied occupation (that would only be terminated later, as a result of the provisions of the
Two Plus Four Treaty), but the city's administrative merger and inclusion in the Federal Republic of Germany, effective on 3 October 1990, had been greenlighted by the Allies, and were formally approved in a meeting of the Allied Control Council on 2 October 1990.
Get it now? The peace treaty wasn't signed by ALL parties until 1990