Mongolia is a one state country in Asia between the Soviet Union, Tannu Tuva, The Japanese puppets in China and the Chinese Warlords. It has mountains in the west, deserts in the south and forests in the north.
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 itself starts off as a very weak nation, and you want to complete the industry branch of your focus tree as fast as possible. You can use your alliance with the Soviet Union to take out many northern warlords. After you take out Xibi San Ma and Sinkiang, you have a good base to build up your army and industry. You can choose at this stage to do the fascist branch of your tree and break away from the soviets, but doing this too early will result in you being unable to beat the Chinese warlords. Sticking with the communists is also a good way to slowly pick off more land in China,