The Xi'an Incident, previously romanized as the Sian Incident, was a political crisis that took place at the Huaqing Pool complex in Xi'an, Shaanxi in 1936. Chiang Kai-shek, leader of the Nationalist government of China, was detained by his subordinate generals Chang Hsüeh-liang (Zhang Xueliang) and Yang Hucheng, in order to force the ruling Chinese Nationalist Party (Kuomintang or KMT) to change its policies regarding the Empire of Japan and the Chinese Communist Party (CCP).
Prior to the incident, Chiang Kai-shek followed a strategy of "first internal pacification, then external resistance" that entailed eliminating the CCP and appeasing Japan to allow time for the modernization of China and its military. After the incident, Chiang aligned with the Communists against the Japanese. However, by the time Chiang arrived in Xi'an on 4 December 1936, negotiations for a united front had been in the works for two years. The crisis ended after two weeks of negotiation, in which Chiang was eventually released and returned to Nanjing, accompanied by Chang. Chiang agreed to end the ongoing civil war against the CCP and began actively preparing for the impending war with Japan.
Background
Japanese invasion of Manchuria
In 1931, the Empire of Japan escalated its aggression against China through the Mukden Incident. Japanese troops then occupied Northeast China. "Young Marshal" Chang Hsüeh-liang, who had succeeded his father as head of the Fengtian clique and Northeastern Army in that region, was widely criticized for this loss of territory. In response, Chang resigned from his position and went on a tour of Europe.
Chang's father had been assassinated by Japan in 1928. Over time, Chang came to view Chiang as ignoring the threat of Japan because of Chiang's focus on opposing the Communists.
Nationalist-Communist conflicts
In the aftermath of the Northern Expedition in 1928, China was nominally unified under the authority of the Nationalist government in Nanjing. Simultaneously, the Nationalist government enacted the Shanghai massacre and purged members of the CCP in the Kuomintang, effectively ending the alliance between the two parties. Beginning in the 1930s, the Nationalist government launched a series of campaigns against the CCP. In the meanwhile, the impending war against Japan led to nationwide unrest and surge of Chinese nationalism. Consequently, the campaigns against the Communist Party were becoming increasingly unpopular. Chiang, fearing the loss of Kuomintang leadership in China, continued the civil war against the CCP despite lacking popular support.
By late 1935 the Communists had narrowly avoided destruction on their Long March and had begun to establish themselves in a new base area on the border between Gansu and Ningxia provinces. They were besieged by a number of nationalist armies, including the Northwestern Army under Yang and the Northeastern Army, to which Chang was re-assigned as commander after his return from a tour of Europe. The Nationalist armies initially gave no notice to the Communist exhortations for war against Japan, but this began to change because of the Red Army's "eastern expedition" from February to April 1936. The Communists declared that they were sending a detachment through Shanxi to fight the Japanese in Rehe and Hubei. Letting the Red Army through would have broken the encirclement, so Yan Xishan stopped them by force. Although defeated militarily, the Red Army had convinced the Shanxi peasantry of their patriotism and gained 8,000 new recruits on their retreat. Chang was likewise impressed and began to see them as potential allies rather than foes. When Mao announced on March 14 that the Communists were willing to conclude a truce, Chang covertly agreed. He proposed to Chiang Kai-shek that he reverse the Nationalist policy of prioritizing the purge of Communists, and instead focusing on military preparation against Japanese aggression. After Chiang refused, Chang began to plot a coup in "great secrecy". By June 1936, the secret agreement between Chang and the CCP had been successfully settled.