We need to know about the historical boundary of the three countries.

During the Eastern Han Dynasty, except for the Central Plains, China's other territories had not been developed, and it was still a deep forest. The northwest terrain is mostly plateau, which is not conducive to farming and sparsely populated. The tropical and subtropical climate and environment in the southwest and south were not suitable for fighting at that time. Therefore, the Central Plains countries did not launch a military war against the surrounding areas. As for China's vast territory, it is basically not caused by military war, but by population migration and gradual improvement. In ancient times, in order to compete for economic value, no one was unfavorable to farming.