I didn't really do a good job of asking my question, but you answered it anyway. Substitute "carpet bombing" for "ethnic cleansing" (and functionally, there ain't a whole lotta difference) and we're very close.
Right-ho. (Actually, there is a lot of difference; carpet bombing kills maybe 10% of the target population, whereas ethnic cleansing kills or displaces over 50%.)
The "raise the drawbridge behind me" approach is only a good plan if the barbarians don't storm the castle. If we basically go isolationist (or, to drag a little less "connotation" into it, let's call it "non-interventionist"), will the folks we're no longer stomping be driven to bring the war to our doorstep in order to maintain their local relevance? "Nope, we're not hiring freedom fighters anymore now that the Evil Imperialists have gone home, but I hear our subsidiary, the terrorist camp down the road, is looking for a few good men so you might drop off your resume."
They won't storm the castle. What they might do, though (and yes, that's a "might," not a "will") is make low-level mayhem on your home ground. If that happens, you'd have to get used to the occasional suicide bomber going off here and there. That, obviously, isn't pleasant, but it's not a serious threat to our way of life either; lots of countries have (and do) put up with it for very long periods of time. You've been lucky to be physically so far from the hotspots that you've never experienced a sustained terror campaign on your home ground, which is why you overreacted so badly to the individual attacks that you did experience.
It can also be fairly effectively combated through traditional counterterrorism tactics -- intelligence, police work, and occasional military strikes against big, stationary targets such as training camps. Genocidal tactics won't work in this kind war either -- there are just too damn many people to kill. You'd find yourself having to wipe out entire populations of tens of millions of people, and even with nuclear weapons, that's not feasible (not to mention wildly disproportionate; it'd put Hitler's Endlösung to shame).
What's more, I believe it's highly likely that that campaign would slowly wind down too, once the grievances fueling it are gone.
In a nutshell, we're just going to have to adjust to a world that's less safe -- unless political and social developments make it safer again. The role of the military, intelligence, espionage, and police operations will be to thwart as many of the attacks as possible, but B2's or no, they can't actually solve the problem by themselves.
