直皖戰爭
In 1919 Xu assumed command of the Northwest Frontier Defense Army, which invaded newly independent Outer Mongolia in October. On November 17 he forced Outer Mongolia to withdraw its declaration of autonomy, thus temporarily bringing Mongolia back under Chinese control. In 1920, after Duan fell from power, Xu lost his position and moved his forces back to confront his enemies. He was replaced in Mongolia by Chen Yi, and Mongolia became independent again in early 1921 when Chinese forces were defeated by the Russian–Mongol army commanded by General Baron Roman von Ungern-Sternberg. Xu's forces were defeated in the subsequent Zhili–Anhui War and he was forced to take shelter in the Japanese embassy.