The period before Japanese-US military cooperation saw Japan controlling most of China, so kind of hard to believe it's served them well.

I would argue the opposite. US backing of Japan has allowed them to maintain an insular chauvinism relative to their neighbors that has put them in weak diplomatic and economic position when faced with China. If they had instead become more tightly coupled with their neighbors like Korea and Taiwan, they’d be in a much better position to check the power of a rising China (think Germany within the context of the EU).

Of course this isn’t entirely Japan’s fault, as the US funded and backed right-wing nationalists in just about all of these countries, so they could play them off each other as was useful to their interests.

> Japan will have to find a way to get along with China

It'll be very difficult to "get along with China"... China (PRC) is the New Thug Nation

Look at China building "island bases" on South China Sea (ironic name, because the area is bordering Vietnam/Malaysia/Philippines)

China's trying to claim what isn't 100% its own out of greed, even though the bordering countries are protesting

https://www.youtube.com/watch?v=luTPMHC7zHY