The People's Republic of China was quickly thrust into its first international conflict soon after being established in June 1950, when North Korea forces crossed the 38th parallel and invaded South Korea. In response, the United Nations, led by the United States, stepped in to defend the South. Thinking a US victory would be dangerous in a time of the Cold War, the Soviet Union left China the responsibility of rescuing the North Korean regime. The US 7th Fleet was sent to the Taiwan Straits to prevent a Communist invasion of the island, and China warned that it would not accept a US-backed Korea on its border. After the UN forces liberated Seoul in September, the Chinese army, known as the People's Volunteers, responded by sending troops south to prevent UN forces from crossing the Yalu River area. Despite the Chinese army's lack of modern warfare experience and technology, the Resist America, Aid Korea Campaign managed to push the UN forces back to the 38th Parallel. The war was costly for China, as more than just volunteers were mobilised and casualties greatly outnumbered those of the UN. The war ended in July 1953 with a UN armistice, and although the conflict had ended, it had effectively prevented the possibility of normalised relations between China and the United States for many years. In addition to the war, China also annexed Tibet in October 1950, claiming it had been nominally subject to the Chinese emperors in centuries past.