Say you have some incalcitrant foreign country that you need to control. I don't think invasion will work, because what do you do then? When the invasion is over, it's over, and you'll have to do something else. What you have to do is occupy the country, and govern every inch of it, all the time. For this you will need thousands of administrators and a vast bureaucracy willing to spend a career there. But even that won't work, because you will end up with a satrapy, whose owner class will eventually be driven out by the natives. So what you have to do is colonize the country, bringing in millions of citizens from the mother country who are willing to commit themselves and their children to life in the colony, working in the fields and factories. But even that won't work, because if the colonies are prosperous, then no doubt they will eventually demand their independence.

I can't think of anything else.