I think it is true. You're right that you can't hold an empire with only force, but I would argue that was just as true during the days of the Roman empire as

I think it is true. You're right that you can't hold an empire with only force, but I would argue that was just as true during the days of the Roman empire as it is today. Sure, the Romans used (for the time) overwhelmingly advanced technology and amazingly brutal tactics to conquer Gaul and Britain, but they maintained their hold over those territories by culturally assimilating the native populations. Gallic and Britanic Romans spoke Latin, wore Roman dress, and even followed Roman sanitary customs -- there's a reason there's a town called Bath in southwestern England. That culture continued to exert influence long after Rome fell as a central military power.