Answer
The Red Army-Mongolian rebel invasion was ultimately responsible for Ungern's defeat. The Red Army, led by Semyon Budyonny, launched a major offensive against Ungern's forces in the winter of 1921-22. The Mongolian rebels, led by Damdin Sükhbaatar, also launched attacks on Ungern's forces. The combined forces of the Red Army and the Mongolian rebels were too much for Ungern to handle, and he was eventually defeated.