The US efforts in Iraq will definitely fail. Societies do not become liberal democracies overnight, but only through an arduous history of amassing wealth, during which time they slowly learn to raise hands in voting instead of breaking them. Autocracy is the only chance for stability in Iraq. The autocracy might come from one of two sources: either a strongman like Saddam or from external—Iranian—control. Even religious authority is not an option in the multi-confessional Iraq.
So far the US arranged for a truce with Iran, which temporarily scaled down its support for Iraqi guerrillas. The US also persuaded the Kurds to refrain from openly seceding from Iraq, and the Iraqi government to refrain from dismantling the de facto Kurdish state. Even if all the major Iraqi groups root out their terrorists, the groups will still have plenty of opportunities to fight among themselves.
Iraq’s mirror is Lebanon: a religiously fragmented state oscillating between fragile, frightful balance and bloody chaos. The Iraqi situation is made worse by the low probability of its government doing away with terrorists completely. As the Israeli example shows, even a few dozen killed are still a big deal when media inflate the stories. Iraqis are happy now with the decrease in violence, but after a year of quiet they will adjust their expectations and view even small terrorist acts as signs of growing instability.
Iran nourishes its influence throughout the Middle East and Africa, and it is unlikely that Iran will abandon the perfect opportunity to influence Shiite Iraq. The Shiites would not be happy about the necessity of reaching consensus with the Sunni minority. Too many forces influence Iraq from within and without. It cannot exist as even a remotely liberal, democratic country. It was for a reason that Iraq never existed before the British created that Frankenstein.
Iraq will become an unofficial Iranian province, fall to a strongman, or fall apart.