we were fighting mass casualty civil wars with complex logistics and annoying steppe niggers when rome was still a nascent city state; conquest is a pain in the fucking ass, and forcing compliance on people without full cultural integration just means you're giving them the keys to your eventual destruction. look at what happened to the british empire, wealthy brown paki or jeet immigrants learn british culture and language and open the way to import their slaves to take over the uk. if we exclude outright military conquest, language and cultural barriers work better than any wall or citadel, because they prevent invaders from coopting your society to sustain themselves.
Canada