Background of Xi'an Incident
Zhang Xueliang, known also as The Young Marshal, was the son of Zhang Zuolin warlord of Manchuria in northeast China. For sometime before the KMT-led China-uniting Northern Expedition, the elder Zhang was being quietly supported by the Japanese government. When it became imminent that the advancing Expedition forces would defeat Zhang and thus threaten Japanese interests in Manchuria, rogue elements within the Kantogun (Japan's Army in Manchuria) forcibly halted the Expedition at Ji'nan and assassinated Zhang on the grounds that he was an unreliable ally, hoping to capitalise on the confusion caused by his death. They miscalculated however, and his son quickly pledged his allegiance to Chiang Kai-shek, turning his forces over to KMT control and supported Chiang in his war of unification against other warlords such as Li Zongren, Feng Yuxiang and Yan Xishan. As a reward, Zhang remained ruler of Manchuria and even extended his influence to Northern China around Beijing and Hebei. Following the Mukden Incident of 1931, when the Japanese invaded and took full and direct control over Manchuria, forcing Zhang, his army and all other Chinese to evacuate, the public placed the responsibility of the disaster on Zhang, who suffered great humiliation in China. By 1936 then, his father's assassination and the loss of his homeland made Zhang into an ardent opponent of the Japanese.
Zhang left China for military training in Europe. After his return, Zhang and his Northeastern China Army were sent to Anhui and Hubei to suppress the Red Army of the Chinese Communist Party. The CPC was forced on the Long March after suffering heavy losses and then set up another base in Yan'an, Shaanxi. Zhang and his troops were transferred to Shaanxi again for suppression in 1936, where he worked with General Yang Hucheng, who used to be a general of Northwestern China Army and a favorite of Feng Yuxiang but later defected to Chiang’s camp.
Zhang and Yang suffered great losses in their attempted suppression of the CPC, and Chiang did not give them any support in manpower and weaponry. It was quite natural for them to think Chiang would take advantage of CPC’s resistance to eliminate their own armies, which were not of Chiang’s own Whampoa Clique. Zhang and Yang began to contact the CPC secretly, and overtly agreed with while covertly opposing Chiang’s policies. Zhang and Yang reached an agreement with CPC for temporary peace. CPC even sent many members to work for Yang.
At the same time, the tension between China and Japan rose day by day. Japan was hoping to conquer China in its entirety by invading vast areas of Northern China. Japanese troops fought against KMT troops along the Great Wall in 1933. Then in 1935, under the accord signed between He Yingqin, the commander of KMT armies in Northern China, and Yoshijiro Umezu, the commander of Japanese troops in Northern China, elite KMT troops related to the group Blue Shirts Society, core of Chiang’s Whampoa Clique, had to evacuate from Beijing and Northern China, which put the whole of Northern China under direct threat of Japanese invasion. But Chiang preferred to unite China by eliminating the warlords and CPC forces first. Chiang believed that he was still too weak to launch an offensive to chase out Japan and that China needed time for a military build-up. Only after unification would it be possible for the KMT to mobilize a war against Japan. So he would rather ignore the discontent and anger among Chinese people at his policy of compromise with the Japanese, and urged Zhang and Yang to carry out suppression efficiently.
Meanwhile, Stalin and his Soviet Union in the 1920s and early of 1930s stood by Japan’s invasion of China at first, for they had also invaded Manchuria and waged a war against Zhang and his father. The Soviets were hoping to make their own territorial gains at the expense of China, dividing it with the Japanese as they would do later with Germany over Poland in Europe. Soon the Soviets became wary of the Japanese ambition and success, fearing it might hurt Soviet interests in the Far East. Then, Stalin began to favor a stronger Chinese resistance to Japan.
Under the authorization of Stalin and Comintern, the delegation of CPC to Comintern led by Wang Ming issued a manifesto urging Chinese to set up a new united front against the Japanese, which was later called the Ba Yi Xuan Yan. In this manifesto, Wang acknowledged that the archenemy of CPC at the present stage was Japan instead of Chiang. But this received a cold shoulder from Mao Zedong and his associates, who ruled CPC and greatly disagreed with Chiang's policies.
These were the complicated situations of, and relationships between, the domestic and foreign parties which preceded the incident.