Just before the Marco Polo Bridge Incident officially touched off the second Sino-Japanese war, which is often grouped together with World War II, China was officially led by Chiang Kai-Shek. However, Chiang had been fighting the Chinese Communist Party for many years at the same time as he had been fighting Japanese encroachment onto Chinese territory. Chiang's leadership as head of the KMT, and in fact all non-Qing leadership after 1911, was accepted by China primarily because it needed leadership that would resist Japan, which had been extremely aggressive since the end of the first Sino-Japanese war some forty years earlier.
Unfortunately for Chiang, he was doing a rather poor job of resisting the Japanese. His policy of Japanese resistance also included a number of provisions for his continued power in China, chief among them being his wish to utterly destroy the power of the CCP before turning China's military strength to the task of repelling the Japanese. Certain other of his advisors disagreed with this tactic, especially since the CCP sought the exact same goal as Chiang's republican KMT government, namely the expulsion of the Japanese from Chinese territory.
The CCP and KMT had been allied with one another before, forming a "United Front" (the official name of the alliance) against the Japanese. However, Chiang had gotten greedy and had had his troops execute a surprise attack on a Communist detachment, bringing the first United Front to an end. Chiang was seen as the only man who could possibly repel the Japanese, being in charge of the most legitimate army in China, but he was also seen as acting inordinately and foolishly in his own self-interest. Instead of eliminating the Japanese, who had annexed some important parts of North China, like Manchuria, and were ruling there with a fair degree of brutality, Chiang was spending his time fighting someone who'd just as soon be his ally in a common fight.
So when Chiang went in December 1936 to Xi'an to confer with one of his top lieutenants, Zhang Xueliang, he ordered Zhang to take the army in the northwest part of the country (near where the CCP's base had been reestablished after the Long March) and reorganize it to begin once again eliminating Communist resistance. Zhang objected, but was ordered to do it anyway, whereupon Zhang threw his cares to the wind and kidnapped Chiang Kaishek on December 12, 1936.
Zhang immediately took effective control of the KMT government and declared the establishment of a second United Front. Since he was a military commander first and foremost rather than a party official, it was fairly certain that some other KMT party member would take power before too long. Unfortunately, that leader would be likely to be more conservative than Chiang and in fact less likely to resist the powerful Japanese. So, even though the Chinese Communist Party's leaders at first called for the public execution of the inept warlord, Mao Zedong, head of the CCP and future head of Communist China, stepped up to ask for his release. At first this was surprising, but with further consideration, it became clear that Chiang Kaishek was, in fact, the only leader capable of kicking the Japanese out of China. He was probably the only KMT willing to do it, and the KMT still enjoyed more general support throughout the majority of China than did the CCP. So on Christmas Day, Chiang was released to retake control of China. Zhang willingly turned himself over to the KMT government and was sentenced to ten years in prison for the kidnapping, a rather light sentence from a regime that later would think nothing about shooting people in the back of the head in the middle of the street.
The United Front would last about another two and a half years, until the enmities of the two sides got the better of them. The war with Japan lasted another nine years, during which Chiang not only proved his own incompetence at keeping his people safe but in fact proved to be a force that the Chinese people should fear, apart from the Japanese. The Japanese began the Rape of Nanking a year and a day after Chiang's kidnapping, but the loss of life there, as brutal as it was, failed to outdo the loss of life and livelihood that occurred when Chiang's own troops broke dikes on the Yellow River in the hope of impeding a Japanese advance a year after that, omitting to inform the people who lived and worked in the flood zone. It's difficult to get behind a ruler when his measures of warfare hurt you as much as the enemy's, and when Chiang Kai-Shek was forced to take refuge in Taiwan some thirteen years later, he was left to do nothing other than lament his total loss of the people's faith.
All information in this writeup is taken either from the lectures of professor Dingxin Zhao or from Jack Gray's excellent book, Rebellions and Revolutions: China from the 1800s to the 1980s.
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