Warsaw Pact
In July 1963 the Mongolian People's Republic asked to join the Warsaw Pact under Article 9 of the treaty. For this purpose a special protocol should have been taken since the text of the treaty applied only to Europe. Due to the emerging Sino-Soviet split, Mongolia remained on observer status. Soviet stationing troops were agreed to stay in Mongolia from 1966.
In which year were Soviet forces scheduled to leave Mongolia? 
1966
Soviet stationing troops were agreed to stay in Mongolia from 1966.
Mongolia