In 1930, when warlords Feng Yuxiang and Yan Xishan attempted to overthrow Chiang Kai-shek's Kuomintang government, Chang stepped in to support the Nanking (Nanjing)-based government against the Northern warlords in exchange for control of the key railroads in Hopeh (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 Mukden (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 Hopeh-Honan (Henan)-Anhui and later in the Northwest.