With the fall of the Qing dynasty in 1911, Mongolia under the Bogd Khaan, (Mongolia's civil and religious ruler) declared independence. But, the newly established Republic of China considered Mongolia to be part of its own territory. After the 1915 tripartite Kyakhta Agreement, Russia, RoC and Mongolia agreed that Mongolia was autonomous under Chinese Control.
In 1919, after the October Revolution in Russia, Chinese troops re-occupied the country due to fears of communism spreading to mainland China. On October 1920, The White Russian General Baron Roman von Ungern-Sternberg led his troops into Mongolia, defeating the Chinese forces in the capital city of Ulaanbaatar. There, Ungern's eccentric, often violent treatment of enemies and his own men, earned him the sobriquet "the Mad Baron". In February 1921 he expelled Chinese troops from Mongolia and restored the monarchic power of the Bogd Khaan.
In June 1921, Communist Mongolian Partisans aided by Soviet Russia established control over parts of the country and on July 6 The Bolsheviks would defeat the white army forces and seize the country for themselves, establishing a communist regime aligned with the Soviets.
Mongolia starts out as a very weak nation, with very little manpower. As of the Waking the Tiger DLC, the Soviets have no interest in demanding Chinese terrirory during peace conferences, so make sure to call them into your first few wars against the Chinese warlords and Communist China. You want to start by justifying against Shanxi and then Communist China ASAP. This ensures you will be able to take them out BEFORE the Chinese United Front forms, and minimise the chance of the soviets signing a non-agression pact with China. Once Shanxi is defeated, you should take all states. Once Communist China capitulates, you want to puppet them, and return the Shanxi territory to them. From this point on, any Chinese states you conquer should be handed to Communist China, giving you some of that sweet, sweet unlimited chinese manpower. At about this point, you want to flip fascist, and continue the game playing as a formiddable Asian power.
Soviet Union, 
Tannu Tuva, 
Sinkiang, 
Xibei San Ma, 
Mengkukuo & 
Manchukuo. It has mountains in the North & West, plains in the East and deserts in the south.
Republic of China considered Mongolia to be part of its own territory. After the 1915 tripartite Kyakhta Agreement, Russia, 
Mongolia, lacking a unique national focus tree, uses the generic national focus tree instead.
Communist nation with no elections.
Democratic
Fascist
Non-Aligned
Limited Conscription
Civilian Economy
Export Focus
