Mongolia-Russia border

The Mongolia–Russia border (Mongolian: Монгол-Оросын хил, Mongol-Orosîn hil; Russian: Российско-монгольская граница, Rossijsko-mongoljskaja granica) is the international border between the Russian Federation (CIS member) and Mongolia. It is virtually all land. The total length of the border is 3485 km. The boundary is the third longest border between Russia and another country, behind the Kazakhstan-Russia border and the China-Russia border.

The Russian state expanded into the regions north of today's Mongolia in the 17th century. Much of the line of the today's Mongolia–Russia border line was set by the Treaty of Kyakhta (1727) between the Russian and Qing Empires (the Mongolia at the time being part of the Qing state); however, the treaty left Tuva on the Chinese (Qing) side of the border.

Mongolia's northern border assumed its nearly modern shape in 1911, as Tuva was separated from Mongolia in the breakup of the Qing Empire (see Mongolian Revolution of 1911), and soon became a Russian protectorate. Although an independent Tuvan People's Republic was declared in 1921, this small country became fully annexed into the Soviet Union in 1944, whereupon the former Mongolia–Tuva border became a section of the Mongolia–USSR border. The latter stayed stable for the rest of the USSR's existence, and continued as the Mongolia–Russia border after the dissolution of the USSR in 1991.

The eastern and western end points of the Mongolia–Russia border are "tripoints", i.e. junctions with the China–Russia border and the China–Mongolia border. A special trilateral agreement, signed on January 27, 1994 in Ulaan Baator, determines the location of these two "tripoints". The agreement is based on earlier bilateral treaties between the parties involved.

...
Wikipedia