Mongolian Revolution of 1911
On 1912 November 3, Russia signed a deal 《俄蒙协约》with Mongolia to mine in Mongolia in exchange for saying Mongolia is autonomous. China was not happy. So in 1913 September 18, Sun Baoqi (孙宝琦) signed a new agreement between China and Russia《中俄声明文件》. China agrees not to put troops in Mongolia and recognize the Russian-Mongol deal 《俄蒙协约》. In return, Russia recognizes that China has suzerainty over Mongolia. So China's power over Mongolia changed from sovereignty to suzerainty.