One of the least understood aspects of the "Surge" and the "improved security," such as it is, is that American forces have allied with groups that not only were insurgents previously (and could become so again), but that these groups are adamantly opposed to the current government set up by the American occupation authority and supposedly free elections (which most Sunnis boycotted).
From the perspective of the Sunni Awakening groups and the neighborhood Guardian groups now cooperating with US forces, the current government is not only illegitimate, but has yet to pay its "blood debt" for killing so many family members of the Awakening and Guardians, and for driving them from their homes and regions. It is because of the Awakening and the Guardians that al Qaeda extremists are hard-pressed, have abandoned much of their former territory and are fighting for their very survival. It is not because of the Surge, except that the additional American troops have increased the ability of these groups to go after al Qaeda. But it is the alliance with the Americans that has made possible the sharing of intelligence, and cooperation. Further, through SERP funds, local US commanders have been subsidizing these groups. They have also pressed the Iraqi government to recognize them.
The Maliki government has made noises about official recognition and pay, and about merging the Sunni forces with their security troops, but it hasn't happened, and it turns out that the government is seen as "intransigent" by both the Sunnis and the Americans. There are very good reasons for this intransigence, however, when you factor in the animosity (and blood feuds) between government and former insurgent forces.
So, now, American soldiers are fighting alongside Iraqis who have sworn bloody vengeance on not just the Mahdi Army (Jaish al Mahdi, or JAM in American troop parlance), but on Iraqi security forces and Iraqi Army forces, as well.
Two questions arise from this: one, who will the American forces support when the Sunni Awakening/Guardians forces confront Iraqi government forces; and two: if the US is not supporting the "democratically elected" government, then what is it there for?
The conclusion to be drawn from all this is that the US is now not only in "the midst of a simmering civil war," but that it is playing one side off against the other, which leads us back to question number two: what are we keeping our troops there for?
Of course the most likely answer is: to control the oil for US companies' profits, since the longer the Iraqi government is in stalemate or reduced to ineffectuality, the more oil will be contracted through the aspiring independent regions: Kurdistan and Shiistan, the former already giving oil companies the kind of contracts that Iraqi nationalists are trying to block, ones that give virtual control to the companies for 20-40 years; can Shiistan be far behind?
For more on the specifics on this see:
Iraq's Civil War Murk