Following the anti-Qing revolution in October 1911, both Outer and Inner Mongolia declared their independence. Inner Mongolia's independence was short-lived, however; it was retaken by the new Chinese leaders, the Guomindang (GMD, Nationalists), who proclaimed the Republic of China (ROC) on 1 January 1912. GMD troops also tried to reoccupy Outer Mongolia but were driven out in 1921, a result of the Russian Bolsheviks' assistance to the Outer Mongolian independence cause. On 11 July 1921 the Outer Mongolians regained their independence and in 1924 proclaimed the Mongolian People's Republic (MPR), with power resting in the hands of the Soviet-sponsored Mongolian People's Revolutionary Party (MPRP). The country was renamed Mongolia, as it is known today. Backed by the Soviet Union, Mongolia's independence was twice reaffirmed by the successors of Qing China: in January 1946 by the ROC and in October 1949 by the PRC, removing the Chinese threat of redeeming Mongolian sovereignty.
Owing to its geographical location, Mongolia's foreign policy direction was entirely shaped by the Sino-Soviet relationship. During the period of Sino-Soviet harmony throughout the 1950s, Mongolia was able to maximize its national interests. Through the 1946 Mongolian-Soviet Treaty of Friendship and Mutual Assistance and the 1952 Sino-Mongolian Agreement on Economic and Cultural Cooperation, Mongolia gained a vast labor supply as well as economic, material and technical assistance. With Soviet approval, Mongolia followed the PRC's lead in building up its international status. In 1955, Mongolia pledged to support the PRC's Five Principles of Peaceful Coexistence, first publicized in the Bandung Conference, in developing diplomatic relationships with noncommunist developing-world nations. In 1961, Mongolia gained United Nations (UN) membership.
As the Sino-Soviet split crystallized in the early 1960s, Mongolia's buffering role was enhanced when the PRC and the Soviet Union competed over the country. In 1960 the PRC concluded with Mongolia the Treaty of Friendship and Mutual Assistance, and in 1962 the China-Mongolia Border Treaty delimited formal boundaries and resolved territorial disputes. On the other hand, the Soviet Union renewed and replaced the 1946 treaty with a twenty-year treaty in 1966. This treaty contained a defense clause, entitling Mongolia to Soviet defense assistance in case of an external invasion, implicitly referring to the PRC. Mongolia chose to side with the Soviet Union, owing in part to the Soviets' record of assistance in its independence movement and in part to the renewed fear of a PRC push to reclaim the lost Outer Mongolia, a perception stemming from the Cultural Revolution (1966–1976).
In accordance with the 1966 treaty terms, in 1967 the Soviets began to deploy troops in Mongolia with the aim of shifting the Soviet defense line southward along the Mongolian-PRC border. When the Sino-Soviet split culminated in a serious border incident in March 1969 along the Amur and Ussuri Rivers, the Soviet Union had already stationed a total of four divisions of ground troops (two tank and two motorized) and unspecified air force units in Mongolia. Some of them were equipped with intermediate-range ballistic missiles with nuclear and chemical warheads, targeting the PRC's nuclear installations in the western province of Xinjiang. Although the border conflict ended in September 1969, the likelihood that Mongolia would become a battlefield for a Sino-Soviet nuclear confrontation remained high. The Soviet Union continued its deployment in Mongolia. In 1970, a coordinated air system was created in Mongolia to control about 1,000 Soviet combat aircraft stationed there. This military presence ensured Mongolia's consistently pro-Soviet and anti-PRC foreign policy until the mid-1980s, when Soviet leader Mikhail Gorbachev decided to restore Sino-Soviet relations and withdraw troops from Mongolia. Free of Soviet pressure, Mongolia normalized its relationship with the PRC at the end of 1986 and established normal ties with the United States in January 1987.
In terms of domestic politics, Gorbachev's political and economic liberalism unleashed the MPRP's political opponents, who championed more democracy. Inspired by the Tiananmen Square protests of spring 1989, the prodemocracy and anti-MPRP Mongolians staged a series of demonstrations, beginning in December 1989 and ending in the MPRP's fall from power in March 1990. In July 1990, a multiparty election was held that created a coalition government.
In January 1992 a new constitution came into force, replacing the People's Republic with a hybrid parliamentary-presidential state. The new constitution forbade the presence of foreign troops, ensuring the true independence and sovereignty of Mongolia.
Christian, David. A History of Russia, Central Asia, and Mongolia. Oxford, UK: Blackwell, 1998.; Kotkin, Stephen, and Bruce A. Elleman. Mongolia in the Twentieth Century: Landlocked Cosmopolitan. Armonk, NY: Sharpe, 1999.