From another perspective, those peripheral lands that have been conquered, are they really securely in the grasp of an empire? In fact, they are not.
Throughout history, both in China and abroad, every conquest signifies brutal exploitation and oppression. Whether it's the assholes who had it easy violating Huaxia or the German leader's frenzied expansion across Europe, the essence is the same.
They had no intention of turning these lands, trampled by their iron hooves, into their territory with kindness, nor did they plan to treat the native inhabitants well, developing them into their own governed people.
These places became exploited colonies. For the United Kingdom, it was North America; for France, it was North Africa.