Marc Lynch provides  some sober analysis of recent events in Iraq:
The absence of U.S. troops because of the 2011 withdrawal is an extremely minor part of the story at best. The intense interaction between the Syrian and Iraqi insurgencies is certainly an important accelerant, but again is only part of the story. Nor is the U.S. reluctance to provide more arms to “moderate” Syrian rebels really the key to the growth of ISIS in Syria or in Iraq. It’s a bit hard to believe that the jihadists who have joined up with ISIS would have been deterred by the presence of U.S.-backed forces – “Well, we were going to wage jihad to establish an Islamic State in Syria and Iraq, but the U.S. is arming moderates so I guess we’ll stay home.” In reality, the shift to an externally fueled insurgency and the flow of money and weapons to a variety of armed groups is what created the conditions that allowed ISIS to thrive in the first place.
Many of the most common reactions to the recent gains made by the Islamic State in Iraq and Syria (ISIS) have been badly mistaken. Complaints that the U.S. “failed” to retain a residual force in Iraq conveniently ignore that the Iraqi government and people were against allowing this, so that was never a realistic option. They also overlook that a continued U.S. military presence would likely not have been able to prevent ISIS’ recent territorial gains, but would almost certainly have provoked a new insurgency that targeted American soldiers. It is extremely doubtful that a small U.S. force would have given Washington any meaningful leverage to force Maliki to change the way that he governs. Maliki was already governing in a sectarian and semi-authoritarian manner when the U.S. had a major military presence in the country, so it seems clear that retaining a smaller presence would have had no effect on him and his allies. It is even more doubtful that the U.S. would use this leverage if it had it.
This is the trouble with trying to condition  future aid on improvements in Maliki’s behavior: when push comes to shove, the U.S. usually refuses to cut off aid because it doesn’t want to “abandon” its client. We trick ourselves into thinking that propping up the client is extremely important to us, which is somehow supposed to justify his abuses and our endless enabling of them. The client knows this and continues to behave however he pleases. Lynch points out that Maliki will probably agree to all sorts of concessions now in order to acquire the aid he seeks, but will forget all about this once the immediate crisis is over:
It will be virtually impossible to force any meaningful political moves in the midst of an urgent crisis, and any promises made now will quickly be forgotten once the crisis has passed.
The Iraqi military has failed to resist ISIS because so many of the soldiers in it have no desire to fight for Maliki’s government, and that is at least partly a product of the abusive nature of his rule. The U.S. wasn’t able to change any of that when our forces were occupying the country, and it won’t be able to change it now. Sending more military equipment to a government that evidently cannot keep the equipment it already has from falling into the hands of its enemies is folly. That incidentally reminds us that sending arms to one approved group in a war zone doesn’t guarantee that those weapons won’t fall into the wrong hands. Sending more weapons into Syria could end up unwittingly aiding ISIS or similar groups. If the Iraqi army can’t keep control of the equipment and weapons the U.S. provided, why would we want to risk the same outcome with the “moderate” rebels in Syria?
Intervening militarily to prevent further advances by ISIS would commit the U.S. to acting as Maliki’s protector indefinitely, and the more resources that the U.S. commits to this the harder it will be to pull the plug at some point in the future. It would also put us in the extremely awkward and politically untenable position of fighting on the same side as the Iranian forces that have already been deployed  to aid Maliki. Having spent years decrying the expansion of Iranian influence in the region (which was aided by the original invasion and overthrow of Hussein), why would interventionists think that we ought to start fighting on the Iranians’ side in Iraq?
It’s true that the U.S. is responsible for wrecking Iraq, and without the invasion and occupation none of this would now be happening. However, it should also be obvious that the U.S. cannot “fix” or even significantly ameliorate the political problems in Iraq through military aid or the use of force. It is imperative that we remember that when we hear the inevitable demands for “action.”