How did Chinese troops regain control of Mongolia in 1919?

In 1919, Chinese troops regained control of Mongolia by occupying Niislel Khüree (now Ulaanbaatar) and deposing the Bogd Khan, who had served as the head of state since Mongolia declared its independence following the collapse of Qing rule.12

This occupation occurred despite the Soviet Russian government's recognition of Mongolian autonomy in August 1919. The Chinese presence was relatively short-lived, however, as the occupation forces were later driven out by tsarist cavalry units led by Baron Roman von Ungern-Sternberg in early 1921, who subsequently restored the Bogd Khan to the throne.2