Hello, I really like this question because the answer reveals how the colonization of a greatly contested area by a great power was able to be undertaken in the context of post-World War One diplomacy, and how the international mechanisms put in place to stop such a thing from happening were ineffective despite unanimous opposition to it.

I should preface this by saying that my main sources for the timeline are the Lytton Report and the Judgement of the International Military Tribunal for the Far East (IMTFF) which occurred after the war. It is worth noting that the former was critiqued by Japanese sources at the time as being biased towards Chinese interests and intentionally anti-Japanese. The latter has both Japanese and some Western scholar (John Dower comes to mind) critics who question the methods of the mostly US-dominated process. Nonetheless, the Lytton Report is still an important source for modern scholarship on the topic and the IMTFF contains quotes of Japanese leadership that give insight into inner workings that weren’t known publically at the time. I also use some critiques of the Lytton Report written by Japanese and Manchurian sources at the time.

What was Manchuria before the Mukden Incident and subsequent Japanese invasion? You point out that the region known as Manchuria located in present-day NE China, (known in Chinese as the three eastern provinces: Liaoning, Kirin, and Heilongjiang) was under the control of Zhang Xueliang. Xuelinag’s father Zuolin had renounced his allegiance to the central Chinese government in 1922 and ruled the region independently while competing with the government and other warlords for control. It should be noted, however, that rather than secession from China, Zuolin characterized his actions as an internal conflict for control of China.

The elder Zhang received aid from Japanese sources who thought it in their interest to not have to deal directly with China. These feelings quickly dissipated, however, as Zhang pursued a competitive approach against the Japanese South Manchuria Railway with the Chinese-owned Chinese Eastern Railway. Further alienation from Japan and cooperation with China by Zhang resulted in his assassination by the Japanese Kwantung Army in 1928. His son, Xueliang, either from a greater sympathy with China, or resentment towards the Japanese, reallied Manchuria to the central government.

Relations between Japan and Zhang continued to decline until the staged Mukden Incident on September 18, 1931, which justified the Japanese invasion of the area. Japan’s invasion and establishment of control over the area were completed in a matter of months. Mukden, the political center of Manchuria, was the first city taken control of. Other central locations were later occupied in the name of protecting Japanese citizens and property.