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).
The Marco Polo Bridge incident did indeed set off WWII. Had it not taken place, something else *may* have, but that is a what-if scenario. There was a political reward in the form of a power shift with each successive grab, as the Japanese legislators responded to each perceived threat to Japanese citizens living in Manchuria by sending more cash. The Japanese had begun to settle there around the turn of the century.
Bringing us into the war — an event triggered by the Japanese attack on Pearl Harbor — led to the annihilation of the Axis regimes and their leaders. There was no upside to it — the Japanese couldn’t have landed troops on the west coast and marched across to our industrial centers. US factories built ships (mostly for us), planes (over 300K of all types, for us and all of our allies), tanks and other other vehicles (over 30K Shermans). After the war, even Stalin (figuratively) toasted the US industrial output.
Meanwhile, the Japanese had loads of raw materials in their newly acquired territories, but starting in 1942 the smallish US sub fleet was shifted to sinking their shipping. The US figure I’ve seen was, 8 million tons sunk, much of it by 1944, but the Japanese figure I’ve seen is 12 million tons. I’d guess that the discrepancy arises from the US War Department’s overcautious assignment of credits for sinkings.