The September 18th Incident was the inevitable result of Japanese imperialism's long-term policy of aggression and expansion against China. It was also an important step taken in an attempt to turn China into its exclusive colony.

Background

According to previous treaties, Japan owned the South Manchuria Railway in Northeast China and the right to garrison troops along the railway. In order to expand its interests, Japan actively advocates the construction of railway branch lines. In the early days, Zhang Zuolin agreed to Japan's construction of some branch lines in order to obtain loans from the Japanese. Later, under pressure from public opinion, Zhang Zuolin planned to build his own railway to break Japan's monopoly.

After Zhang Zuolin was assassinated in the Huanggutun Incident in June 1928, his son Zhang Xueliang continued his father's policy, which aroused dissatisfaction in Japan. On December 12 of the same year, Zhang Xueliang complied with the Northern Expedition, announced the change of banner in Northeast China, and obeyed the Nanjing National Government.

On March 26, 1929, the Northeastern Kuomintang Party Headquarters was established in Fengtian and advocated the recovery of the Dalian Concession in Lushun and the South Manchuria Railway. This caused panic in Japan and was the pending case of the Northeastern Railway.

In May 1930, the Central Plains War broke out in the Pass. In September, Zhang Xueliang led the main force of the Northeast Army out of Shanhaiguan to support Chiang Kai-shek's Central Army. After the victory, he led the troops to stay in Yanjing, leaving a void outside the Pass.

In July 1931, the Wanbaoshan Incident and the Nakamura Incident occurred in the Northeast. Zhang Xueliang instructed the Northeast to exercise restraint and avoid conflicts with the Japanese army.