Decision on the Capital of Germany

The capital resolution (German: Hauptstadtbeschluss) was made by the German Bundestag on 20 June 1991, as a result of German reunification, to move its headquarters, as well as the headquarters of half the federal ministries, from Bonn to Berlin. Ministries which remained in Bonn, such as the Federal Ministry of Defence, established secondary offices in Berlin; likewise the seats in Bonn turned to second offices for ministries which moved to Berlin.

Source: Wikipedia — Decision on the Capital of Germany (CC BY-SA 4.0)