After German reunification, the Federal Republic of Germany and Poland sign a treaty confirming the Oder–Neisse line as the border between Germany and Poland.

German reunification, also known as the expansion of the Federal Republic of Germany (BRD), was the process of re-establishing Germany as a single sovereign state, which began on 9 November 1989 and culminated on 3 October 1990 with the dissolution of the German Democratic Republic and the integration of its re-established constituent federated states into the Federal Republic of Germany to form present-day Germany. This date was chosen as the customary German Unity Day, and has thereafter been celebrated each year as a national holiday. On the same date, East and West Berlin were also reunified into a single city, which eventually became the capital of Germany.