began. However, the Red Army showed a higher combat capability, and the conflict ended with the signing of the Khabarovsk Protocol of December 22, 1929. In 1930, when warlords Feng Yuxiang and Yan Xishan attempted to overthrow Chiang Kai-shek's Kuomintang government, Chang stepped in to support the Nanjing-based government against the Northern warlords in exchange for control of the key railroads in Hebei and the customs revenues from the port city of Tianjin. A year later, in the September 18 Mukden Incident, Japanese troops attacked Chang's forces in Shenyang in order to provoke a full-on war with China, which Chiang did not want to face until his forces were stronger. In accordance with this strategy, Zhang's armies withdrew from the front lines without significant engagements, leading to the effective Japanese occupation of Zhang's former northeastern domain. There has been speculation that Chiang Kai-Shek wrote a letter to Chang asking him to pull his forces back, but Zhang later stated that he himself issued the orders. Apparently, Chang was aware of how weak his forces were compared to the Japanese and wished to preserve his position by retaining a sizeable army. Nonetheless, this would still be in line with Chiang's overall strategic standings. Chang later traveled in Europe before returning to China to take command of the Encirclement Campaigns, first in Hebei-Henan-Anhui and later in the Northwest. Xi'an incident On April 6, 1936, Chang met with CPC delegate Zhou Enlai to plan the end of the Chinese Civil War. KMT leader Generalissimo Chiang Kai-shek at the time took a passive position against Japan and considered the communists to be a greater danger to the Republic of China than the Japanese, and his overall strategy was to annihilate the communists before focusing his efforts on the Japanese. He believed that "communism was a cancer while the Japanese represented a superficial wound." Growing nationalist anger against Japan made this position very unpopular, and led to Chang's action against Chiang, known as the Xi'an Incident. In December 1936, Chang and General Yang Hucheng kidnapped Chiang, imprisoning him until he agreed to form a united front with the communists against the Japanese invasion. After two weeks of negotiations, Chiang agreed to unite with the communists and drive the Japanese out of China. When Chiang was released on December 26, Chang chose to return to the capital city of Nanjing with him; once they were away