The Chinese Civil War began in 1927, when the Nationalist Guomindang (Kuomintang) leader Chiang Kai-shek purged his Communist allies in Shanghai. The next twenty-two years would settle the future of the world's most populous country between two organised political projects. The 'Long March' of 1934–35 — the retreat of besieged CCP forces some 9,000 km from southern China to the north-west — saw only about ten thousand of the original hundred thousand survive. The march consolidated Mao's position inside the party and became its founding myth. The Japanese invasion of 1937–45 briefly united the two sides, but no real partnership emerged; the CCP organised in the countryside on the promise of land reform while the Guomindang ran an exhausted war administration from the cities.

After Japan's surrender in 1945 the war resumed. The final phase of 1946–49 proved decisive: despite American material support, Guomindang rule collapsed under hyperinflation, corruption, and the disintegration of its officer corps. Mao's peasant army moved south from Manchuria, taking Beijing in early 1949, Nanjing in April, Shanghai in May. The Chiang Kai-shek government withdrew to the island of Taiwan with much of its army and treasury — the divided sovereignty that still defines the strait today. On 1 October 1949 Mao proclaimed the People's Republic from the rostrum at Tiananmen Square with the words 'The Chinese people have stood up.' Within weeks the Soviet Union recognised the new regime; other states followed gradually, while the United States went on recognising the Taiwan government as 'China' until 1979.

The revolution rapidly turned into a destructive programme of forced development. In 1950–53 China sent troops to the Korean War, drawing it into indirect conflict with the United States. The Great Leap Forward, launched in 1958 — a utopian campaign to mobilise peasants for steel production — combined with agricultural collapse to produce the famine of 1959–61, in which most estimates place between 15 and 45 million deaths, the largest loss of human life of the modern era. The Cultural Revolution of 1966–76 closed universities, sent millions to the countryside, and purged Mao's rivals; estimates of the dead range from hundreds of thousands to several millions. After Mao's death in 1976 the Deng Xiaoping reforms began; the gradual market opening from 1978 onward made China the world's second-largest economy within a single generation.

The 1949 Revolution is one of the most decisive events of the twentieth century. It gave post-colonial Asia a model of communist transformation that Vietnam, Korea, and later revolutionary movements drew on. At the same time, the catastrophes of social engineering during its first three decades — the Great Leap Forward, the Cultural Revolution — set the cost ledger of that revolution. Today's China carries two contradictory inheritances from this history: the fastest rise from poverty to global power on record, and a continuous human-rights argument under single-party rule. Mao's proclamation of 1 October 1949 is at once a declaration of independence and the beginning of three decades of severe upheaval.