On September 18, 1931, several packages of blasting powder were detonated on the South Manchuria Railway. The Kwantung Army blamed the Chinese for this, and used it as an excuse to invade Manchuria, launching a series of events which would culminate in the rise of militarism in Japan.
SOURCES:
Japan's Total Empire, Young
Ishiwara Kanji's Confrontation with the West, Peattie
The Japanese Colonial Empire, Peattie
The Making of Japanese Manchuria, MatsusakaGo Fast Imperialism: The Manchurian Incident & Interwar Japanese Colonial Politics
The Historian's Craft | October 1, 2021
I believe China took great pains under the Communists to move Han Chinese into Manchuria and other outlying areas, so that they wouldn't pose a breakaway nationalist threat to China.
The Manchurians were such a small group compared to the Chinese that it's surprising that they were able to conquer the country and hold onto it for centuries.
Just as a lone assassin didn’t trigger WW1, the Manchukuo incident didn’t set off WW2. Japan had begun seeding the region with its people, after 1905 (Russo-Japanese War). It was a “sphere of influence” — the object of an imperialist approach. There were factions in Japan, pro and con. Democracy v militarism, aristocracy v militarism. The militarists didn’t have it their way just yet.
And over a million Japanese in Manchuria took severe punishment after the war, from not only the Chinese but the Soviets as well, who did not behave as civilly as the occupiers of Japan proper.
The puppet state was not even the first overreach by Japan, but at the time the Japanese militarists considered it the essential reach, with mineral resources and Port Arthur.
Once the militarists were firmly in charge, late 1930s, they had another problem: the minerals and the ports weren’t going to do them much good without oil, rubber, and shipping access points like the Strait of Malacca.
The history of this time and region is fascinating. A truly bone-headed policy by a nation scarcely out of medievalism, feeling its oats, bent on modernity and parity with powerful nations, yet without the cultural foundations necessary. Indeed as their technology and ambitions advanced, their culture regressed (Shintoism, samurai and emperor worship).
At least they gave Pu Yi something to do with his spare time.