The Bush administration left the Obama administration with a thorny problem in Iraq, namely how to withdraw without leaving behind a region inexorably and irretrievably in the grip of Iran as the region's preeminent power.
As Stratfor's George Friedman noted months ago:
It is now April, meaning we are four months from the deadline for the completion of the withdrawal of U.S. combat forces from Iraq. In the balance is not only Iraq, but also the Iranian situation. What happens next all comes down to whether the mass of parties in Baghdad share a common foundation on which to build a nation — and whether the police and military would be loyal enough to this government to die for it. If not, then the entire edifice of U.S. policy in the region — going back to the surge — is not merely at risk, but untenable. If it is untenable, then the United States must craft a new strategy in the region, redefining relationships radically — beginning with Iran.
As with many things in life, it is not a matter of what the United States might want, or what it might think to be fair. Power is like money — you either have it or you don’t. And if you don’t, you can’t afford to indulge your appetites. If things in Baghdad work themselves out, all of this is moot. If things don’t work out, the Obama administration will be forced to make its first truly difficult foreign policy decisions.
Now, the drawdown date for US combat forces is upon us and the Iraqi's do not yet have a functional government. If they cannot be relied on to bring their country together and hold it against overbearing Iranian influence, President Obama will be faced with very difficult choices, one of which may be negotiation with Iran. Such a thing could be a diplomatic tour de force akin to Nixon going to China, or a debacle.