Outer Mongolia was once part of China, and there is no doubt about that. During the Qing Dynasty, the Kangxi Emperor incorporated the Kalku Mongols in northern Mobei into the territory of China. However, from the end of the Qing Dynasty, Outer Mongolia gradually moved towards independence.
Outer Mongolia refers to the Mongol tribes north of the desert that were late in alliance with the Qing Dynasty, and because they were attacked by the Dzungar Khanate, they allied with the Qing Dynasty to jointly deal with the Dzungars, and then officially submitted to the Qing Dynasty.
In particular, Kangxi and Yongzheng affirmed that Mongolia was under the jurisdiction of the Qing Dynasty through the signing of a treaty with Tsarist Russia.
But we know that a paper agreement is nothing more than a piece of waste paper for aggressive Russians. Therefore, the Russians continued to infiltrate the Mongolian region, and even in the unequal treaties signed between Tsarist Russia and the Qing government, a large number of provisions involving Outer Mongolia were included. It can be seen that Tsarist Russia's ambitions for foreign Mongolia are clearly revealed.
Just before the fall of the Qing Dynasty, the feudal upper echelons of Outer Mongolia led by Jebtsundamba, with the support of Tsarist Russia, declared the "independence" of Outer Mongolia and established the "Great Mongolian Government", not only expelling Qing officials, but also signing the illegal Russian-Mongolian Entente with Tsarist Russia.
The Qing government certainly did not agree, but the situation could not be controlled.
In 1913, the Beiyang government and Tsarist Russia reached the "Sino-Russian Declaration Document", which exchanged the suzerainty of Outer Mongolia at the cost of recognizing the autonomy of Outer Mongolia. Of course, we know that Tsarist Russia does not want to give up its rights and interests in the northeast, but it is already a big victory to let Outer Mongolia cancel its independence and implement autonomy.
But Tsarist Russia then became busy and could not take care of Outer Mongolia, especially the backyard was on fire, and the Soviets overthrew Tsarist Russia. Without foreign aid, Foreign Mongolia sent a telegram to the Beiyang government in Beijing, requesting that "independence" be abolished and that it return to China's embrace.
But I am afraid that no one has thought of the Soviet Era, so the technique has been repeated. Although The Soviet Union faced internal and external troubles, it did not stop its wolf ambitions to make Outer Mongolia independent.
After the establishment of Soviet Russia, it exported revolution to Mongolia and fully supported the establishment of the People's Party army in Outer Mongolia, and there is no doubt that Outer Mongolia once again became independent, and then with the support of the Soviet Union, the constitutional monarchy was abolished and the Mongolian People's Republic was established.
After that, Outer Mongolia has become de facto independent. Although neither the Beiyang government nor the later Nationalist government recognized it, it could no longer influence the development of the situation.
At the end of World War II, in order to win the Soviet Union against Japan, the Allies signed the Yalta Agreement with the Soviet Union without informing China, one of which was to maintain the status quo in Outer Mongolia.
The Nationalist government then signed a "Sino-Soviet Treaty of Friendship and Alliance" with the Soviet Union, which was a new unequal treaty, and on the issue of Outer Mongolia, the Soviet Union never gave up, so the Nationalist government could only swallow this bitter fruit.
From the late Qing Dynasty to the Republic of China period, China has been in a chaotic situation, there is no time to take care of, nor can it control this independent tendency, although it has refused to admit it many times, but it is not capable, and finally Outer Mongolia is completely independent under the control of the Soviet Union.