Tsarist Russia was formerly a small European country, but it was still part of the Grand Duchy of Moscow. In the 13th century, the Mongols lived on the European Asian continent and were the overlords at that time, and the Mongols completed the conquest of Western Xia, Dajin, Western Liao and other countries under the leadership of Genghis Khan. The Mongols made three European expeditions, the second of which came directly to the Eastern European Plain and established the Golden Horde, which at that time directly controlled parts of Siberia and Eastern Europe. The Grand Duke of Moscow began to accept the jurisdiction of the Golden Horde and to commission tax collection. After 240 years of Mongol rule, the Grand Duchy of Moscow broke away from its independence and embarked on the path of foreign expansion. In the 17th century, Tsarist Russia expanded to Siberia, and Tsarist Russia encountered the Qing Dynasty in the process of expansion. The Qing Dynasty was also a regional power at the time. At that time, the Qing Dynasty was also facing internal troubles, and discussed with Tsarist Russia to resolve the eastern and central boundaries.
Golden Horde. 
Grand Duchy of Moscow.
In 1689, Tsarist Russia and the Qing Dynasty signed the Treaty of Nebuchu, according to which the dividing line between the Qing Dynasty and Tsarist Russia was the Trans-Khing'an Mountains and the Wudi River, which legally determined that the Qing Dynasty had sovereignty over the Heilongjiang River basin and the Ussuri River basin. In 1728, the Qing Dynasty signed the Treaty of Kyakhta with Tsarist Russia, in which Tsarist Russia had control of the land of Lake Belga, and Outer Mongolia was the territory of the Qing Dynasty, which was also recognized by Tsarist Russia at that time. In the middle of the 19th century, when the Qing Dynasty was facing internal and external troubles, Tsarist Russia used force to demand that the Qing Dynasty sign an unequal treaty, and in 1858, Tsarist Russia and the Qing Dynasty signed the "Aihui Treaty", and Tsarist Russia ceded more than 600,000 square kilometers on the north bank of the Heilongjiang River Basin. In 1860, the Qing Dynasty signed the Sino-Russian Treaty of Beijing with Tsarist Russia, in which the Qing Dynasty ceded more than 400,000 square kilometers of land on the east bank of the Ussuri River. Tsarist Russia also set the goal of expansion in Outer Mongolia. Tsarist Russia first sent trade troops to Outer Mongolia and actively maintained close ties with the Outer Mongolian leadership. Tsarist Russia was also active in providing funds to the Mongol nobility,** and training personnel. Tsarist Russia also began to emigrate to Outer Mongolia and began to buy land in the local area, etc.
Treaty of Nebuchu.
Tsarist Russia ceded the territory of the Qing Dynasty.
In 1911, the Wuchang Uprising broke out within the Qing Dynasty, and the Qing Dynasty faced a crisis of rule. Seeing this opportunity, Tsarist Russia immediately agitated for Outer Mongolia to become independent from the Qing Dynasty. Tsarist Russia sent troops directly into Outer Mongolia, which had already turned Outer Mongolia into its sphere of influence. In 1915, Tsarist Russia and the Beiyang authorities signed the "Sino-Russian-Mongolian Agreement", Outer Mongolia belongs to Chinese territory, Outer Mongolia has the right of independent autonomy, and Tsarist Russia can trade in Outer Mongolia, build roads, open mines and other rights. In 1917, when the October Revolution broke out in Tsarist Russia, the Beiyang authorities sent troops into Outer Mongolia and stationed them directly in Outer Mongolia. In 1924, the Soviet Union and the Beiyang authorities signed the Agreement on the Settlement of Suspense, under which the sovereignty of Outer Mongolia belonged to China and the autonomy of Outer Mongolia remained independent. The Soviet Union also stationed troops in Outer Mongolia and secretly supported the establishment of an autonomous republic in Outer Mongolia. In the 30s of the 20th century, the Soviet Union openly provided the Mongols with troop training and provision, and signed a protection treaty with the Mongols. In 1938, in order to expand in Outer Mongolia, Japan broke out a war with the Soviet army at Nomenkan, which made the Japanese army voluntarily abandon the invasion of Soviet land and set the goal of expansion in Southeast Asia.
In 1945, Soviet leader Joseph Stalin, British Prime Minister Winston Churchill and Roosevelt jointly issued the Yalta Declaration, which first maintained the Middle East Railway and subsidiary railways in the Northeast under the control of the Soviet Union. The third place leased the Port Arthur Group to the Soviet Union as a military port, and developed the Port of Dalian into an international port. Fourth, the USSR gained sovereignty over the southern part of Sakhalin and the Kuril Islands. With this treaty, the Soviet Union took direct control of Outer Mongolia until 1991. Outer Mongolia has always aspired to join the Soviet Union, as a result of which Outer Mongolia was recognized by the Soviet Union as a producer of animal husbandry. Outer Mongolia was relatively backward in economic development and needed significant economic assistance during the Soviet era.
After the collapse of the Soviet Union in 1991, Outer Mongolia was freed from Soviet control, and Outer Mongolia openly declared the need to carry out third-neighbor diplomacy, and Outer Mongolia actively interacted with the United States, Japan, South Korea, NATO and other Western countries and organizations. Since Outer Mongolia is bordered by Russian Siberia to the north, Western countries have always used the location of Outer Mongolia to directly threaten the strategic value area of Russian Siberia. Mongolia has also been actively following the path of Westernization and accepting the ideas and cultures of Western countries. Russia is actively courting Mongolia, and Mongolia's important strategic areas will directly make Russia have to show economic interests to win over Mongolia.
The Mongols have always wanted to get rid of the Eastern Dragon Kingdom and the polar bear, but they are surrounded by two countries on all sides, and the nearest country is Kazakhstan, which is 38 kilometers away, and Russia is currently attacking the former Soviet republics, and if Outer Mongolia directly threatens the interests of Russia on the island, Russia will choose to directly send troops to control Mongolia. Mongolia has a land area of 1.56 million square kilometers and a population of only 3.3 million. The Mongols are vast and sparsely populated. If you control Outer Mongolia and control the capital Ulaanbaatar, you can control most of Mongolia. During the Soviet era, the Mongolian region could be turned into the sixteenth republic of the Soviet Union, and the Mongolian region was too poor and needed a lot of subsidies during the Soviet period.
Mongolian topography.