In 1949, the Communist Party of China won the Chinese Civil War in mainland China and established the People's Republic of China (PRC), claiming to be the sole legitimate government of China. The ROC government retreated to the island of Taiwan (which it gained control of in 1945 at the end of hostilities in WWII), Quemoy Island, and the Matsu Islands. Until 1991, the ROC also actively claimed to be the sole legitimate government of China, and during the 1950s and 1960s this claim was accepted by the United States and most of its allies. While the PRC was an ally of the Soviet Union, the United States sought to prevent the Communist bloc from gaining another permanent seat in the Security Council. To protest the exclusion of the PRC, Soviet representatives boycotted the UN from January to August 1950, so they didn't veto the intervention of UN military forces in Korea.