The warring states were all part of a more-or-less unified preexisting Chinese culture, sharing sacred objects, rites, language, etc. It's not ridiculous to see the quest to take control of China as a different sort of thing from the quest to take control of foreigners.

If you believe the analysis in https://www.amazon.com/Early-Chinese-Empires-History-Imperia... , the Qin dynasty immediately flamed out because its institutions just didn't work in peacetime, and the Han dynasty had to develop practices more suited to a stable empire.