110 years ago today, on June 7, 1915 (April 25, 1915 lunar calendar), China, Russia and Mongolia signed a treaty. Chinese representative Chen Yu took a group photo with Russian and Mongolian representatives at the meeting in Chaktu. On June 7, 1915, the Sino-Russian-Mongolian treaty was signed in Chaktu, and China lost its actual sovereignty in Outer Mongolia. According to the 1913 Sino-Russian declaration document and annexes, the interests of China and Russia in Outer Mongolia should be determined by China, Russia and Mongolia at a location and appointed representatives to contact. Therefore, on September 8, 1914, the three parties started negotiations in Chaktu. Since September 8, 1914, it has been signed and established for 9 months. Forty-eight formal meetings were held, and no less than 40 meetings and negotiations were held before the conclusion. The delay of the meeting was due to the serious differences between China and Russia and Mongolia, such as Mongolia's declaration that there is no independent matter, the cancellation of the imperial name and the "common wear" era name, the railway in Outer Mongolia, the postal service, the number of guards for the Chinese in Kulun and their assistant commissioners, the judicial proceedings in Outer Mongolia, and the Mongolian border tax rules. Due to the wide gap between the two sides' positions, China once intended to withdraw the negotiation, but the Chinese government was well aware that China was unable to recover Outer Mongolia by force, and the "21-Article" requirements between China and Japan were becoming more and more serious. It had to compromise and signed the "Sino-Russian-Mongolian Entente", a total of 22 articles. China only retained the name of suzerainty.