So one of the long-term problems in Iraq has been (and will be) the difficulty in standing up a "national" army instead of a sectarian one. Iraq, of course, had a national army that the United States deliberately destroyed. But whatever.
Here's the problem for the US, and any other occupying army, anywhere: foreign countries have no legitimate claim to nationalism. It's kind of obvious, and it shouldn't even need to be said, but we will never, ever be able to tell the Iraqis what's good for their country, especially as long as our "advice" comes at the business end of an Abrams tank.
But suppose we succeeded in creating, from the ether, a truly national Iraqi army. What would it's first order of business be? To ask the US to leave. Because they don't intend to do so, the second order of business would be to force the issue. Ergo, America can never succeed in creating an effective, national army in Iraq.
The problems go deeper. The most nationalistic people in Iraq will not, in any possible future, volunteer in large numbers to serve in the collaborating regime propped up by the great Satan. If drafted, they will betray their units to other forces -- such as the Sadr JAM. The most nationalistic Iraqis, if they exist in the sense that we mean that word, are already fighting the occupation (and have been for years) in the hopes that their sectarian force will come out on top once the Americans leave. National unity through victory.
So this leaves the Americans with all the problems of the occupation still, but unable to put a native face on it. Worse still, because the Americans still have military objectives in Iraq, they need to arm various factions -- Sunni, Shia, Kurd -- as auxiliaries. But of course, all this does is arm the three sides of Iraq's eventual civil war.