remained in place, it also made it easier for West Berliners to travel to East Germany and it simplified the regulations for Germans travelling along the autobahn transit routes. At the Brandenburg Gate in 1987, U.S. President Ronald Reagan provided a challenge to the then Soviet leader: General Secretary Gorbachev, if you seek peace, if you seek prosperity for the Soviet Union and Eastern Europe, if you seek liberalization: Come here to this gate! Mr. Gorbachev, open this gate! Mr. Gorbachev, tear down this wall! On 9 November 1989, the Wall was opened, and the two parts of the city were once again physically—though at this point not legally—united. The Two Plus Four Treaty, signed by the two German states and the four wartime allies, paved the way for German reunification and an end to the Western Allies' occupation of West Berlin. On 3 October 1990—the day Germany was officially reunified—East and West Berlin formally reunited as the city of Berlin, which then joined the enlarged Federal Republic as a city-state along the lines of the existing West German city-states of Bremen and Hamburg. Walter Momper, the mayor of West Berlin, became the first mayor of the reunified city in the interim. City-wide elections in December 1990 resulted in the first "all Berlin" mayor being elected to take office in January 1991, with the separate offices of mayors in East and West Berlin expiring by that time, and Eberhard Diepgen (a former mayor of West Berlin) became the first elected mayor of a reunited Berlin. Boroughs West Berlin comprised the following boroughs (Bezirke): In the American Sector: Neukölln Kreuzberg Schöneberg Steglitz Tempelhof Zehlendorf In the British Sector: Charlottenburg Tiergarten Wilmersdorf Spandau In the French Sector: Reinickendorf Wedding Exclaves West Berlin's border was identical to the municipal boundary of Berlin as defined in the Greater Berlin Act of 1920 and amended in 1938, and the border between the Soviet sector and the French, British, and American sectors respectively, which followed the boundaries of Berlin administrative boroughs as defined in the same years. Another amendment was added in 1945 at the border between the British sector of Berlin (ceding West-Staaken) and the Soviet zone (ceding the Seeburg Salient) so that the Wehrmacht airfield at Berlin-Gatow became part of the British sector and the airfield at Berlin-Staaken became part of the Soviet sector. The resulting borderline was further complicated with a lot of