In December 1911, Outer Mongolia took advantage of the Xinhai Revolution to declare independence from the Qing Dynasty. The political system of new Mongolia was an absolute theocratic monarchy led by Bogd Khan. However, the newly founded Republic of China considered Mongolia as part of its territory. In the 1915 tripartite Kyakhta Agreement, Russia (which had strategic interests in Mongolian independence but did not want to completely alienate China), the Republic of China and Mongolia agreed that Mongolia was autonomous under Chinese suzerainty. However, in the following years Russian influence in Asia waned due to the First World War and, later, the October Revolution. From 1918 on, Mongolia was threatened by the Russian Civil War, and in summer 1918 asked for Chinese military assistance, which lead to the deployment of a small force to Urga. Meanwhile, some Mongolian aristocrats had become more and more dissatisfied with their marginalization at the hands of the theocratic government, and, also provoked by the threat of the Outer Mongolia's independence from the pan-Mongolist movement in Siberia, by 1919 were ready to accept Chinese rule.
The invasion of Mongolia was the brainchild of Premier Duan Qirui. When Duan engineered China's entry into the First World War he took out several large loans from the Japanese government including the Nishihara Loans. He used the money to create the War Participation Army ostensibly to battle the Central Powers. His rivals knew the purpose of this army was to crush internal dissent. It existed outside of the Ministry of the Army and was controlled by the War Participation Bureau which the premier led. The Bureau was staffed entirely by Duan's Anhui clique. President Feng Guozhang, Duan's rival, had no control despite constitutionally being commander-in-chief. When the war ended without a soldier stepping foot abroad, his critics demanded the disbanding of the War Participation Army. Duan had to find a new purpose for his army. Mongolia was chosen for several reasons:
Duan's envoys to the 1919 Paris Peace Conference were unable to prevent the German concession Shandong being transferred to Japan thereby causing the nationalist May Fourth Movement to target his policies. His reputation as a patriot was discredited. Reintegrating Mongolia would reverse that.
The Constitutional Protection War was fought to a bloody standstill in Hunan. Using his army for another risky attempt to retake southern China from the rebels was undesirable.
The Russian Civil War left Mongolia without a foreign protector. An easy victory would boost Duan's stature.
Mongolia's long running prime minister, Tögs-Ochiryn Namnansüren, died in April 1919 leaving the country's ruling elite deeply divided over a successor. Some of Mongolia's princes as well as the ethnic Han community longed for reunification.
The War Participation Army was renamed the Northwestern Frontier Army. Duan gave control of it to his right-hand, Xu Shuzheng, member of the Pro Japanese Anhui Clique in the Chinese government. They announced the expedition was at the invitation of several Mongolian princes to protect Mongolia from Bolshevik incursions. It was supposed to begin in July 1919, but the train broke down. In October, Xu led a spearhead group of 4,000 that quickly captured Urga without resistance. Another 10,000 troops followed to occupy the rest of the country. The successful invasion was met with acclaim throughout China, even by Sun Yat-sen's rival southern government. The Japanese were the ones who ordered the pro Japanese Chinese warlords to occupy Mongolia in order to halt a possibly revolutionary spillover from the Russian revolutionaries into Mongolia and Northern China. After the Chinese completed the occupation, the Japanese then abandoned them and left them on their own.Manlaibaatar Damdinsüren said that "I can defend Mongolia from China and Red Russia".
In February 1920, Xu presided over a very humiliating ceremony in which Bogd Khan and other leaders were forced to kowtow before him and the Five Races Under One Union flag. This event marked the beginning of active resistance against Chinese rule which coalesced into the Mongolian People's Party.
Domestic politics in China soon changed the situation dramatically. The invasion had caused alarm for Zhang Zuolin, the powerful warlord of Manchuria, who was upset that such a large army was moved so close to his territory. He joined the chorus of critics such as Cao Kun and Wu Peifu calling for the removal of the Anhui clique. In July, they forced President Xu Shichang to remove Xu Shuzheng from his position. In response, Xu Shuzheng moved the bulk of his forces to confront his enemies in China. Both he and Duan Qirui were defeated in the ensuing Zhili-Anhui War. This left only a few Chinese troops in Mongolia.
In October, the White RussianBaron R.F. von Ungern-Sternberg swept into Mongolia from the north and fought many battles with the Chinese garrison stationed in Urga before capturing it on February 1921. There he defeated the Chinese forces and restored Bogd Khan as a monarch. At around the same time, the MPP engaged in its first battle against Chinese troops. A joint MPP-Red Army expedition led by Soviet Red commanders and Damdin Sükhbaatar defeated the Baron in August. Tensions leading up to the First Zhili-Fengtian War and the apparent victory of the Bolsheviks in the Russian Civil War led to the end of China's involvement.
This was the last foreign occupation in Mongolian history. A communism-orientated state such as Mongolia was unexpected in orthodox Marxism, and would probably not have happened if there were no occupation. After a brief period of constitutional monarchy, the Mongolian People's Republic was established in 1924 which would last until 1992.
For China, the occupation indirectly led to the permanent breakup of the Beiyang Army and the fall of strongman Duan Qirui. This marked the period of high warlordism as the former officers of Yuan Shikai battled each other for many years to come. Many White Russian guerrillas became mercenaries in China after the occupation. Along with the Siberian Intervention, it was the only foreign military expedition carried out by the Beiyang government. In 2002, however, the Republic of China, which now controls only Taiwan and surrounding islands, announced that it was administratively recognizing Mongolia as an independent country, excluding Mongolia from the official maps of the Republic of China and requiring Mongolian citizens visiting Taiwan to produce passports.Informal relations were established between Mongolia and Taiwan via trade offices in Ulan Bator and Taipei, albeit without formal diplomatic recognition due to the One-China policy, as Mongolia recognizes the People's Republic of China. No legislative actions were taken to address concerns over the Republic of China's constitutional claims to Mongolia, as amending the Constitution of the Republic of China is a politically sensitive issue because of the political status of Taiwan.
^Thomas E. Ewing, "Russia, China, and the Origins of the Mongolian People's Republic, 1911-1921: A Reappraisal", in The Slavonic and East European Review, Vol. 58, No. 3 (Jul., 1980), p. 406ff
^John S. Major (1990). The land and people of Mongolia. Harper and Row. p. 119. ISBN0-397-32386-7. "in 1919, a Japanese influenced faction in the Chinese government mounted an invasion of Outer Mongolia and forced its leaders to sign a "request" to be taken over by the government of China. Japan's aim was to protect its own economic, political, and military interests in North China be keeping the Russian Revolution from influencing Mongolia."|accessdate= requires |url= (help)
^"Mongolian office to ride into Taipei by end of the year". Taipei Times. 2002-10-11. Retrieved 2009-05-28. "In October 1945, the people of Outer Mongolia voted for independence, gaining the recognition of many countries, including the Republic of China. (...) Due to a souring of relations with the Soviet Union in the early 1950s, however, the ROC revoked recognition of Outer Mongolia, reclaiming it as ROC territory."