After World War I The Chinese felt betrayed. Anger and frustration erupted in demonstrations on May 4, 1919, in Beijing. Joined by workers and merchants, the movement spread to major cities. The Chinese representative at Versailles refused to endorse the peace treaty, but its provisions remained unchanged. Disillusioned with the West, many Chinese looked elsewhere for help.

The May Fourth Movement, which grew out of the student uprising, attacked Confucianism, initiated a vernacular style of writing, and promoted science. Scholars of international stature, such as John Dewey and Bertrand Russell, were invited to lecture. Numerous magazines were published to stimulate new thoughts. Toward the end of the movement's existence, a split occurred among its leaders. Some, like Ch'en Tu-hsiu and Li Ta-chao, were beginning to be influenced by the success of the Russian Revolution of 1917, which contrasted sharply with the failure of the 1911 Revolution in China to change the social order and improve conditions. By 1920, people associated with the Comintern (Communist International) were disseminating literature in China and helping to start Communist groups, including one led by Mao Zedong. A meeting at Shanghai in 1921 was actually the first party congress of the Communist Party of China (CCP).

The CCP was so small that the Soviet Union looked elsewhere for a viable political ally. A Comintern agent, Adolph Joffe, was sent to China to approach Sun Yat-sen, who had failed to obtain assistance from Great Britain or the United States. The period of Sino-Soviet collaboration began with the Sun-Joffe Declaration of Jan. 26, 1923. The KMT was recognized by the Soviet Union, and the Communists were admitted as members. With Soviet aid, the KMT army was built up. A young officer, Chiang Kai-shek, was sent to Moscow for training. Upon returning, he was put in charge of the Whampoa Military Academy, established to train soldiers to fight the warlords, who controlled much of China S(See Chiang Kai-shek). Zhou Enlai (also Chou En-lai) of the CCP was deputy director of the academy's political department.

Sun Yat-sen, whose power base was in the south, had planned to send an expedition against the northern warlords, but he died before it could get under way. Chiang Kai-shek, who succeeded him in the KMT leadership, began the northern expedition in July 1926. The Nationalist army met little resistance and by April 1927 had reached the lower Yangtze. Meanwhile, Chiang, claiming to be a sincere follower of Sun Yat-sen, had broken with the left-wing elements of the KMT. After the Nationalist forces had taken Shanghai, a Communist-led general strike was suppressed with bloodshed. Following suppressions in other cities, Chiang set up his own government at Nanjing on April 18, 1927. He professed friendship with the Soviet Union, but by July 1927 he was expelling Communists from the KMT. Some left-wingers left for the Soviet Union.

The northern expedition was resumed, and in 1928 Chiang took Peking. China was formally unified. Nationalist China was recognized by the Western powers and supported by loans from foreign banks.