Annexation of Tibet by the People's Republic of China
Central Tibet came under the control of the People's Republic of China (PRC) after the government of Tibet signed the Seventeen Point Agreement which the 14th Dalai Lama ratified on 24 October 1951. This followed attempts by the Tibetan government to modernize its military, negotiate with the PRC, and the defeat of the Tibetan Army by the People's Liberation Army.
Source: Wikipedia — Annexation of Tibet by the People's Republic of China (CC BY-SA 4.0)