 Complete the following phrase:  The occupation of Outer Mongolia by the Beiyang government of the Republic of China since 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 Ungerns White Russian Buryats, Russians etc. and Mongolian forces. These, in turn, were defeated by the Red Army and its 