Why didn't China conquer the world?

Answer 1

China is the Middle Kingdom, it is a center of wisdom and enlightenment, and everything outside can be considered barbaric. The Chinese learned long ago that tributary nations require as much support as they provide. The Chinese have only had desire to conquer the world for a few fleeting moments in history, usually when the ruling people were not Chinese natives, and then they realized they are far better off being content with what they have.

Answer 2

In the time of Ancient China, there were no powers to overthrow it.
That simple.
Japan was its own country, and did not feel the need to cause war.
Remember; like today, China was huge, powerful, and had a real army (for that time period).
I'm sure many tried to invade China, but their attempts were useless.