What was the purpose of the Japanese to establish Manchukuo?

You only need to look at the interests to understand international affairs.

There are three aspects for Japanese to establish pseudo-everywhere:

First, you can occupy the resources in Northeast China and solve the problem of domestic resource shortage.

Second, I don't want to see a big country next to my own country. Splitting and controlling China is of great significance to the long-term strategy.

Third, by controlling the puppet Manchukuo, it is easier to develop northward and southward. You can even occupy the Chaoxuan Peninsula.

Below, there is a proof:

Japanese Army Minister Nan made a report on "Recent Situation of Manchuria" to the Japanese Emperor, and suggested establishing a "new country" in the northeast of China. Minami Jiro pointed out in his report that it is very convenient for Japan to attack the Soviet Union and "develop northward" in the future. By controlling the economy of this "new country", Japan can obtain the resources that make Japan self-sufficient permanently, and it can also solve the problem of large population but small population by emigrating to the northeast of China.