Japan had begun a war of purely imperial agression against China in the 1930s. Japan didn't even really need to expand into China, its economy was doing relatively well without the war, but conservative, pro-military factions felt they needed a war to maintain their grip on Japanese society.
The Japanese elite didn't fight an imperialist war to maintain their grip on Japanese society; they fought the war to achieve autarky by way of a Greater East Asia Co-Prosperity Sphere. The main lesson that the Japanese took away from WWI was that a state needed to be self-sufficient in terms of resources if it wanted to survive in a modern, total war. They fought WWII in an attempt to gain this self-sufficiency. It's no coincidence that Manchuria and the Dutch East Indies were the main targets of Japanese aggression, for their coal/iron and oil respectively.
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