The occupation of Outer Mongolia by the Beiyang government of the Republic of China after the revocation of Outer Mongolian autonomy began in October 1919 and lasted until 18 March 1921, when Chinese troops in Urga were routed by Baron Roman von Ungern-Sternberg's White Russian and Mongolian forces. These, in turn, were defeated by the Red Army and its Mongolian allies by June 1921.

The Beiyang government was the internationally recognized government of the Republic of China between 1912 and 1928, based in Beijing. It was dominated by the generals of the Beiyang Army, giving it its name.