Thomas Ricks, a guest commentator on NPR today and author of the book "The Gamble," argued that if U.S. troops leave today that there would be genocide in Iraq. I think he's wrong.
In my view the decline in violence in Iraq is not mostly consequence of a troop surge. It is a consequence of the segregation of Iraq. In most of Iraq, members of minority ethnic groups of fled either abroad or to other regions of Iraq where they are in the majority. Where there isn't segregation at the provincial level, it has happened at the neighborhood level, with minority neighborhoods becoming fortified outposts on the model of Cold War Berlin. Violence has declined because militias have run out of targets in places where they have power. While a Warsaw Ghetto scenario is possible in parts of Baghdad, the civilian leaders in Iraq so no inclination to pick that kind of fight now that their power is secure.
There are a couple of provinces that are still in flux, but provincial elections have laid the ground work for civilian rule, which nationally looks likely to be an Iran friendly regime. The national government, moreover, looks like it is in a position to impose its will upon the disputed regions.
At this point, even a dramatic event, like the break up of Iraq into Kurdish, Shiite and Sunni mini-states wouldn't necessary disrupt regional security very much. The Kurds are de facto independent and have been since before the U.S. invaded in 2003 anyway. Non-Kurdish Iraq doesn't have enough military might to be a threat to its neighbors. Iran can secure control politically, so it has little incentive to do so with violence. Kuwait and Saudi Arabia rely upon the goodwill of the Americans too much to move in militarily in Iraq (for no obvious gain). If Syria intervenes militarily in the Sunni dominated Anbar and Ninevah provinces of Iraq, as it has in Lebanon, and as its huge Iraqi refugee population may encourage it to do, local authorities may welcome the invaders with flowers and heart felt cheers.
Removing American troops, who comprise the last remnants of occupying foreign soldiers, eliminates one of the most important remaining flash points that would motivate violent factions. Even if more foreign troops are needed to maintain stability, it isn't at all obvious that U.S. troops are good candidates for the role.