For the bulk of our troops and our combat responsibilities, one perhaps dominant line of thought is start withdrawing and end our front-line involvement for those that remain. On the one hand, the danger is a ferocious bloodbath and instability that will permeate the region, causing larger, more intractable problems. On the other hand, however, is the quite plausible proposition that this fiasco will play out anyhow according to its own logic and dynamics with the US having precious little influence on the outcome.
Leaving saves our lives and treasure, and frees us to more directly address a comprehensive strategy (a strategy beyond Iraq) in the wider conflict with violent Islamic radicals. Staying will cost us lives, but MAY save some Iraqi lives and give us a chance of preventing regional turmoil featuring, most likely, an open Sunni-Shiite belligerency. But, quite honestly, it might not no matter what we do.