West Germany's relations with East Germany posed particularly difficult questions. Though anxious to relieve serious hardships for divided families and to reduce friction, West Germany under Brandt's "Ostpolitik" was intent on holding to its concept of "two German states in one German nation." Relations gradually improved. In the early 1970s, the "Ostpolitik" led to a form of mutual recognition between East and West Germany. The Treaty of Moscow (August 1970), the Treaty of Warsaw (December 1970), the Four Power Agreement on Berlin (September 1971), the Transit Agreement (May 1972), and the Basic Treaty (December 1972) helped to normalise relations between East and West Germany and led to both states joining the United Nations, in September 1973. The two German states exchanged permanent representatives in 1974, and, in 1987, East German head of state Erich Honecker paid an official visit to West Germany.