On July 9, 1926, Chiang took his 100,000 soldiers of the National Revolutionary Army (NRA), led by students trained in the Whampoa Military Academy and equipped with Russian and German weapons, out on the Northern Expedition. NRA soldiers were far better organized than the warlord armies which they faced due to their military advisers, among whom was the famous Soviet General Vasily Blyukher (code-named Galen). In addition, the NRA was regarded as a liberating force on the side of common people persecuted by warlords, so it received strong support from the peasants. The NRA progressed from the Zhu River area to Yangtze River in less than half a year, defeating the warlords Wu Peifu in the Central Plain region and Sun Chuanfang on the east coast, and grew from 100,000 to 250,000. The main remaining target in the north was Zhang Zuolin, the warlord of Manchuria.

By 1927, KMT forces turned against western imperialist powers, notably Britain. They seized the British concessions in Hankou, but Britain was determined to defend its interests in Shanghai. In the outskirts of Shanghai, a labor union general strike broke out, paralyzing the city. The communist-led unionists embraced the KMT. Chiang, however, formed secret alliances with the financial elite (including his brother-in-law) and the Green Gang in Shanghai to break up strikes and union meetings. Chiang decided to purge all Communists from the Kuomintang, so on April 12 he instituted the Shanghai Massacre.

Trotsky and Stalin The Northern Expedition became a point of contention in foreign policy between Joseph Stalin and Leon Trotsky. Stalin followed an opportunistic policy, ignoring communist ideology. He told the CCP to stop complaining about the lower classes and follow the KMT's orders. Stalin believed that the KMT middle and upper classes would defeat the western imperialists in China and complete the revolution. Trotsky wanted the Communist party to complete an orthodox proletarian revolution and he opposed funding the KMT. Stalin countered Trotskyist criticism by making a secret speech in which he said that Chiang's right wing Kuomintang were the only ones capable of defeating the imperialists, that Chiang Kai-shek had funding from the rich merchants, and that his forces could be squeezed of all benefit then discarded like a lemon. Chiang turned the tables on him, though, with the Shanghai massacre of 1927. 

The Shanghai Massacre

In response to the advances of the NRA, Communists in Shanghai began to plan uprisings against the warlord forces controlling the city. On March 21–22, 1927, KMT and CCP union workers led by Zhou Enlai and Chen Duxiu launched an armed uprising in Shanghai, defeating the warlord forces of the Zhili clique. The victorious union workers occupied and governed urban Shanghai except for the international settlements prior to the arrival of the NRA's Eastern Route Army led by Generals Bai Chongxi and Li Zongren. After the Nanjing Incident, in which foreign concessions in Nanjing were attacked and looted, both the right wing of the Kuomintang and western powers became alarmed by the growth of Communist influence, while the CCP continued to organize daily mass student protests and labor strikes demanding the return of Shanghai international settlements to Chinese control. With Bai's army firmly in control of Shanghai, on April 2, 1927 the Central Control Commission of KMT, led by former Chancellor of Peking University Cai Yuanpei, determined that CCP actions were anti-revolutionary and undermined the national interest of China, and voted unanimously to purge the communists from KMT. 

Arrests and executions begin

On April 5, 1927, Wang Jingwei arrived in Shanghai from overseas and met with CCP leader Chen Duxiu. After their meeting they issued a joint declaration re-affirming the principle of cooperation between KMT and CCP, despite urgent pleas from Chiang and other KMT elders to eliminate Communist influence. 

On April 9 Chiang declared martial law in Shanghai and the Central Control Commission issued the "Party Protection and National Salvation" proclamation, denouncing the Wuhan Nationalist Government's policy of cooperation with the CCP. On April 11 Chiang issued a secret order to all provinces under the control of his forces to purge Communists from the KMT. Before dawn on April 12 gang members began to attack district offices controlled by the union workers. Under an emergency decree, Chiang ordered the 26th Army to disarm the workers' militias. The union workers organized a mass meeting denouncing Chiang Kai-shek on April 13, and thousands of workers and students went to the headquarters of the 2nd Division of the 26th Army to protest. Soldiers opened fire, killing 100 and wounding many more. Chiang dissolved the provisional government of Shanghai, labor unions and all other organizations under Communist control, and reorganized a network of unions with allegiance to the KMT and under the control of Du Yuesheng. Over 1000 Communists were arrested, some 300 were officially executed and more than 5,000 went missing. Some NRA commanders with Communist backgrounds who were graduates of Whampoa Military Academy kept their sympathies for the Communists hidden and were not arrested.

Aftermath and significance

The KMT Central Committee, including Sun's widow Soong Ching-ling, publicly denounced Chiang as a traitor to Sun Yat-sen, immediately after the purge. But Chiang Kai-shek was defiant, forming a new Government at Nanjing to rival the Communist-tolerant Government in Wuhan controlled by Wang Jingwei.

The rival KMT governments did not last long because the Wuhan KMT also began to violently purge Communists after Wang found out about a secret order from Stalin to Borodin to overthrow the left-wing KMT and take over the Wuhan government. More than 10,000 communists were arrested and killed within 20 days. Thousands fled to rural areas or remained underground in cities. The Soviet Union officially terminated its cooperation with the KMT while Wang, fearing retribution as a Communist sympathizer, fled to Europe. Afterwards the Wuhan Nationalist government disintegrated, leaving Chiang as the sole legitimate leader of the Kuomintang. In the year after April 1927, over 300,000 people died across China in anti-Communist suppression campaigns executed by KMT. 

During the remainder of 1927, the Communists launched several revolts in an attempt to win back power, marking the beginning of the Chinese Civil War. With the failure of the Guangzhou Uprising (December 11–13, 1927), the CCP's eclipse was complete; it was two decades before they were able to launch another major urban offensive.

On the Communist side, Chen Duxiu and his Soviet advisers, who had promoted cooperation with the KMT, were discredited and lost their leadership roles in the CCP. Chen was personally blamed, forced to resign, and replaced by Qu Qiubai, who did not change Chen's policies in any fundamental way. The CCP planned further worker uprisings and revolutions in the urban areas. 

The first battles of the ten-year Chinese Civil War began with armed Communist insurrections in Changsha, Shantou, Nanchang and Guangzhou. During the Nanchang Uprising in August 1927, Communist troops under Zhu De were defeated but escaped from KMT forces by withdrawing into the mountains of western Jiangxi. In September 1927 Mao Zedong led a small peasant army in what has come to be called the Autumn Harvest Uprising in Hunan province. It was defeated by KMT forces and the survivors retreated to Jiangxi as well, forming the first elements of what would become the People's Liberation Army. By the time the CCP Central Committee was forced to flee Shanghai in 1933, Mao had established peasant-based soviets in Jiangxi and Hunan provinces, transforming the Communist Party's base of support from the urban proletariat to the countryside, where the People's War would be fought.

In June 1928, the NRA captured Beijing, leading to the nominal unification of China and worldwide recognition of the KMT led by Chiang Kai-shek as the legal government of the Republic.

Warlord counteroffensive

The purge gave the warlords an opportunity to rebuild their armies. Sun Chuanfang began communicating with Zhang Zuolin, warlord of Manchuria. Sun arrayed his forces along the Lower Yangtze, aiming to unleash an all-out attack upon the KMT and regain Nanking. The KMT, divided by political tensions and leadership conflicts, succumbed to Sun’s surprise attack on July 24. His army tore through the surprised KMT forces and took Xuzhou in northern Jiangsu province. Chiang’s armies retreated south and launched a counter-attack in August, resulting in a terrible defeat for the KMT. This defeat led to Chiang's immediate resignation on Aug. 6th as head of the Nanking Government, prompting him to move to Shanghai. Li Zongren moved the whole NRA to the Yangtze to defend Nanking. On August 25, Sun Chuanfang's army, now close to the Yangtze, launched an all-out attack upon the KMT forces at Longtan, a vital supply route between Nanking and Shanghai. Chiang’s forces were initially pushed out, but regained the city on August 30.

Second Northern Expedition

On January 2, 1928 the leftist KMT government in Wuhan finally reconciled with Chiang and formally merged with the Nanking Government. In response to the recapture of Xuzhou by the KMT, Zhang Zuolin recruited Sun’s troops to join his Anguo Army. On April 2, Chiang ordered the beginning of the Second Northern Expedition. The Nationalists swept across the remains of Sun’s forces and reached the Yellow River in mid-April. Meanwhile in Manchuria, Zhang Zuolin was assassinated by conspirators from Japan's Kwantung Army. Due to his hatred of the Japanese, who had killed his father, Zhang Xueliang, the new warlord of Manchuria decided to cooperate with Chiang and the KMT, thus nominally uniting China under one state.

Outcome

The Northern Expedition is viewed positively in China today because it ended a period of disorder and started the formation of an effective central government. However, it did not fully solve the warlord problem, as many warlords still had large armies that served their needs, not those of China. This is because of their alliance with the KMT. They acted as franchisees of the party, wore NRA uniforms, and espoused the party doctrine. Wars between these new warlords would claim more lives than ever in the 1930s and prove to be a major problem for the KMT right up to 1949.

Chiang benefited most from the expedition, however, for the victory achieved his personal goal of becoming paramount leader. Furthermore, Chiang made the military command superior to KMT party leadership, which resulted in his later dictatorship.