If international policy were dictated by decency rather than calculated self-interest, there'd be no need for a state department, or a defense department for that matter.

I agree with you insofar as leaving well enough alone and allowing the region to revert to tribally-defined borders would ultimately be good for people living in the immediate region. Maybe. Once the mass killings subside a bit. It's a little bit mass-killy over there right now for a lot of residents' tastes, I should think. But as far as your other assertion goes- that it would have no repercussions for America- I'm not convinced of that at all. Oh, it's clear that ISIS isn't in the business of farming out overseas attacks a la AQ. But terrorism isn't really the only shark swimming in the sea, is it?

I won't pretend to understand all of our motivations for keeping the Middle East relatively stable on our terms. I don't think it's as simple as oil, oil, oil, though. I suspect it has just as much to do with the number of barely stable states within the region whose access to things that go boom is either well-established or burgeoning. Pakistan and Iran come clearly to mind, but Egypt, Iran, Libya and Syria enjoy either confirmed or suspected CW stockpiles; all of the aforementioned states make some pretty neat missiles in general.

The US might want Iraq stable as much to stave off the messages its collapse would send as much for the benefits it awards us by its continued existence. There's the "US is a paper tiger incapable of supporting the states it builds" thing, and then there's the "if this can happen here, why not in every unstable Middle Eastern state" thing- that one has been borne out in part by the whole Arab Spring affair. The other lesson we can draw from the Arab Spring is that while it's easier than we might have once thought to topple a repressive regime, it's a lot harder to pick up the pieces and glue them together into a coherent whole again, even when all of the actors in play are ostensibly interested in doing so.

Again, this is straight-up speculation. I dunno, MW. Maybe Iraq falls and nothing happens. But from what we've seen elsewhere in the region, nothing really happens in a vacuum, and up to now we don't have many positive outcomes to place our bets by. At very least, I think it's safe to say Iraq's fall would have destabilizing effects on bordermates Syria and Iran, and those are two states that we want propped up come hell or high water.