The area controlled by the Bogd Khaan was approximately that of the former Outer Mongolia during the Qing period. In 1919, after the October Revolution in Russia, Chinese troops led by warlord Xu Shuzheng occupied Mongolia. Warfare erupted on the northern border. As a result of the Russian Civil War, the White Russian Lieutenant General Baron Ungern led his troops into Mongolia in October 1920, defeating the Chinese forces in Niislel Khüree (now Ulaanbaatar) in early February 1921 with Mongol support.