China – Mongolia

The international border separating China and Mongolia is known as the China-Mongolia border. It stretches 4,630 km (2,880 mi) from west to east between the two tripoints with Russia, with the majority of the border territory located in the Gobi Desert. It is the fourth-longest international border in the entire planet. Just 100 kilometers (62 miles) east of the tripoint between China, Kazakhstan, and Russia, in the Altai Mountains, the boundary with Russia officially begins. From then, it travels overland typically in a south-easterly direction, with straight line sections predominating in the Gobi Desert area, until it reaches the southernmost point of Mongolia, which is located just north of the 40°30 line of latitude.

The route then continues overland in a north-easterly direction, passing close to Mongolia's easternmost point in the Greater Khingan mountains. From there, the border turns steeply to the north-east up to the eastern Russian tripoint before veering to the north-west and then west, using the Khalkhyn Gol river for a while until passing into Buir Lake. Since 1949, when the Communists have controlled China, relations with Mongolia have steadily improved. On December 26, 1962, the two nations signed a treaty defining their shared border. Then, between 1963 and 1964, the entire border was demarcated, and on June 30, 1964, a final treaty with a thorough set of maps was reached. The boundary has remained in place, and although there have been some stressful moments since then, most notably during the Sino-Soviet split in the 1960s when Mongolia sided with the USSR, overall ties between the two countries have remained friendly.

Length: 4,630 kilometers