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.
Marco Polo Bridge was an entirely different event. 1931, Mukden incident. 1937, MPB. The latter, most would agree, touched off the Sino-Japanese War (not WW2). The former was the “false flag” to justify invading Manchuria.
The general reader might enjoy David Bergamini’s “Japan’s Imperial Conspiracy.”