Japan's motive was Imperialism; nothing more and nothing less. They wished to control all of Asia from Burma to New Guinea, and North. They would have had Indonesia and Australia too if they were able to pull it off. In plain language, their leaders were BGs.
Their big mistake was that "imperialism" had gone out of fashion in the international community by the early 1920s, with the creation of The League of Nations. Prior to the end of WWI, everyone else was doing exactly what Japan wanted to do---conquer and exploit.
They were 40 years late on the world stage with their plan. It had nothing to do with whatever nascent Chinese communism might have existed before Japan invaded China---almost a decade before the US got involved.
As for the last comment about Chinese ambitions, they were enabled by none other than our president, Richard Nixon. ANd further enabled by none other than our President G. W. I. And still further enabled by
Mr. Clinton and the corporate moguls at Walmart. Ross Perot was about the only person in the business community to call free trade what it is---a big sucking sound.