China–Mongolia border

Outer Mongolia breaks away

China divided the Mongols into Inner and Outer Mongolians. Outer Mongolia had paid tribute to China since 1691, but the Manchu presence there was thin, and by the early 20th century Han settlers were moving onto Mongol pastures. In December 1911, as the Manchu dynasty collapsed, Mongolia under the Living Buddha of Urga declared itself independent of Chinese rule.^[1]

Mongolia turned to Russia as a counterweight. Agreements in 1912, 1913 and 1915 left China a nominal suzerain and Russia the stronger protector; the 1913 declaration admitted there were no detailed maps and that the exact boundaries would be settled later. Chinese troops reoccupied Outer Mongolia in 1919 but were defeated in July 1921 by Mongolian and Soviet forces, and the Mongolian People’s Republic was proclaimed in 1924.^[1]

At Yalta in 1945 the Allies agreed that the status quo in Outer Mongolia would be preserved, and after a plebiscite in January 1946 voted for independence, China recognised the Mongolian People’s Republic. In 1947 China told the UN Security Council that Mongolian troops had pushed 100 km into China around the Baytik Mountains, where the two sides clashed.^[1]

Maps that did not agree

For decades the line existed mainly on rival maps. Russian maps of 1914 stretched Mongolia deep into Xinjiang and Manchuria; a 1943 Chinese series placed a large part of the Mongolian Altai in Xinjiang, while a 1940 Soviet series pushed the frontier south to keep the whole Altai in Mongolia. Comparing the two, analysts reckoned that at least 86,000 km² of the Altai were in dispute.^[1]

After a 1960 visit to Ulaanbaatar by China’s premier, Zhou Enlai, the two countries signed a border treaty in December 1962, part of a series China concluded with Burma, Nepal, Pakistan and Afghanistan. A joint commission demarcated the line in 1964 with 639 pillars and a boundary atlas of 105 map sheets. The US Geographer judged that the settlement favoured the traditional Mongolian version and that China had been quite generous.^[1]

Relations soon cooled. The joint inspection the protocol called for did not meet until February–April 1982, and in July 1981 Mongolia complained of aggressive Chinese actions, while the Soviet Union counted more than 400 border violations by Chinese troops since 1969.^[1]

“Inasmuch as there are no detailed maps of Mongolia and as the boundaries of the administrative divisions of this country are uncertain”^[1]

Mountains, desert and steppe

In the west the border follows the main crests of the Altai at 3,000–4,000 metres, then passes the Baytik Range, where the two countries once disputed about 15,000 km².^[1] East of it lies the Gobi: more than 1,500 km of gravel plains with little surface water, rare settlement and summer and winter temperatures that can differ by more than 40 °C. The Beijing–Ulaanbaatar railway crosses it between Erenhot and Dzamin Uud.^[1]

In the east the line crosses steppe and the Dariganga grazing lands, part of Inner Mongolia until 1924, then bends around spurs of the Khingan Range. Near Lake Buyr and the Halhin (Halaha) River, Soviet and Mongolian forces defeated Japan’s Kwangtung Army in 1939 in fighting over where Mongolia ended and Japanese-run Manchukuo began.^[1]