Sino-Soviet border conflict
With the intensification of the Sino-Soviet border conflict in the 60s, the signing of "The Treaty of Friendship, Cooperation and Mutual Assistance between the USSR and the Mongolian People's Republic" in Ulaanbaatar in early 1966 by Brezhnev and Tsedenbal, allowed the Soviet Union to station troops in Mongolia to ensure mutual defense, being the first time that foreign troops would be stationed in the republic.