Russia had expanded far into
Siberia during the course of the 17th century, bringing it into conflict with
Qing China, which at that time
ruled over Outer Mongolia. Much of the line of the today's Mongolia–Russia border line was set by the
Treaty of Kyakhta between Russia and China. With China engulfed in chaos during the
Xinhai Revolution, Mongolian nationalists
seized the opportunity to declare Outer Mongolia independent from China, with the support of Russia. In 1915 the
Second Treaty of Kyakhta was signed, by which Russia acknowledged formal Chinese
suzerainty over Mongolia, albeit with Russia maintaining significant influence, leaving the country in effect as a semi-autonomous condominium. Following the
Russian Revolution in 1917 China invaded Mongolia in an attempt to
re-assert full control, however they were ultimately repulsed by Mongolian and Soviet Russian forces, with Mongolia once again declaring independence from China in 1921. China's refusal to recognise Mongolian independence meant that no formal border delimitation was conducted, though the remoteness and inhospitable, scarcely populated boundary terrain meant that this was in practice not a pressing issue. However, following
Japan's
invasion of
Manchuria in 1931 and
disputes over the Nomonhan region, Mongolia and the Japanese puppet-state of
Manchukuo delimited a small section of their eastern frontier in 1935–39 in the vicinity of
Buir Lake, though Japan was defeated in the
Second World War leaving the status of this agreement in doubt. Following a
plebiscite, and assurances from the USSR that they would not interfere in China's restive
Xinjiang province, China agreed to recognise Mongolia's independence in 1946. Disputes soon arose over the border, notably over the gold-rich Baytik Mountains in 1947, and further work on boundary delimitation was hampered by the
Chinese Civil War. With the Communists in power in China from 1949, relations with Mongolia steadily improved, and the two countries signed a treaty on 26 December 1962 delimiting their common frontier. A full border demarcation then occurred from 1963 to 1964 and a final treaty with a detailed set of maps agreed upon on 30 June 1964. Though relations have at times been tense since then, notably during the 1960s
Sino-Soviet split in which Mongolia sided with the USSR, the border has remained where it is and relations between the two states have remained generally cordial. ==Sex trafficking==