In 1921, Soviet troops entered the country and facilitated the establishment of a republic by Mongolian revolutionaries in 1924. China also made a claim to the region but was too weak to assert it. Under the 1945 Chinese-Russian Treaty, China agreed to give up Outer Mongolia, which, after a plebiscite, became a nominally independent country.
A 20-year treaty of friendship and cooperation, signed in 1966, entitled Mongolia to call on the USSR for military aid in the event of invasion. Thus allied with the USSR in a dispute with China, Mongolia began mobilizing troops along its borders in 1968 when the two powers became involved in border clashes on the Kazakh-Sinkiang frontier to the west and at the Amur and Ussuri rivers.