"Jiang Jieshi resigned as President on 21 January 1949, and the Communists captured Beijing the following day. They responded to the new President’s offer of negotiation by demanding unconditional surrender, and the war continued. The Communist victories that winter had opened the way to advances further south, not least by enabling them to build up resources. The Communists crossed the Yangzi River on 20 April 1949, and the rapid overrunning of much of southern China over the following six months testified not only to the potential speed of operations, but also to the impact of success in winning over support. Nanjing fell on 22 April, and Shanghai on 27 May, and the Communists pressed on to capture rapidly the other major centres. Fleeing the mainland, Jiang Jieshi took refuge on the island of Formosa (Taiwan), which China had regained from Japan after the end of World War Two. It was protected by the limited aerial and naval capability of the Communists and, eventually, by American naval power. However, until he intervened in Korea in 1950, Mao Zedong prepared for an invasion of Formosa, creating an air force to that end. Jiang, in turn, used Formosa and the other offshore islands he still controlled as a base for raids on the mainland. Meanwhile, in the spring of 1950, the island of Hainan and, in 1950–1, Tibet were conquered by the Communists, the capital of Tibet, Lhasa, being occupied on 7 October 1950. The CIA subsequently backed rebellion in Tibet, notably by Khampa rebels. The new strategic order in Asia was underlined in January 1950 when China and the Soviet Union signed a mutual security agreement. Mao was in Moscow for two weeks, an unheard of amount of time for a head of state: Stalin would not see Mao for a while, insulting him. Tension between the two regimes was there from the start."