The Iraqi Sunni Arabs have run Iraq (or the area that is Iraq) for centuries, and they are very unhappy about losing that power to Shia Arabs (who they see as allied with the hated Iranians, who are not even Arab and have been the dominant military power in the region for over 3,000 years.) Losing power, and control of the oil money, as well as the threat of prosecution for atrocities committed for Saddam, has created enough manpower to keep al Qaeda going in Iraq. But it's a war, and al Qaeda is losing. Day by day, there are fewer towns where al Qaeda can set up shop, fewer neighborhoods free of pro-government Sunni Arabs who will use their newly acquired cell phone to report terrorist activity.
You can't ignore history in the Middle East, because people there have long, and vivid, memories. People will kill to avenge some ancient slight, and that's what's being played out in Iraq. Peace will return when the losers are beaten down by force and negotiation. Most Sunni Arabs have gone over to the negotiation side of things, and basically admitted that the new order is not likely to be changed any time soon. But the hardliners still have al Qaeda and terrorist fantasies of Sunni Arab supremacy in Iraq. The majority of Iraqis, and their foreign allies, won't let that happen. So it's just a matter of time before the shrinking terrorist force is reduced to the level of a minor police problem. That's the history of Iraq and the Middle East. You can look it up.
The Sunnis are gradually coming to terms with the facts of life. The jihadis are foreign invaders who will be hunted down and killed. Syria is not a direct threat and in time, Iraq can deal with Syria if Damascus refuses to behave by cutting off support for the jihadis. I only really worry that Iran might conclude that they can't win with this slow Coalition victory march and decide to roll the dice and hit Iraq with either a pretend Shia uprising actually controlled by Iran or possibly even a conventional invasion of the south. Or a combination.
If nothing upsets the present course, we will win. Will Iran accept defeat in Iraq?