Many commentators seem puzzled over President Obama’s decision to use an Oval Office speech to mark the “end of combat operations” in Iraq. The reason: Iraq is important to Barack Obama, even if most Americans are nowadays preoccupied with a foundering economy.

Iraq, in fact, may be the reason Obama is President. During the 2008 campaign, the very green Junior Senator from Illinois used his opposition to the war to distinguish himself from more experienced rivals like Hillary Clinton and Joe Biden. His anti-war credentials allowed him to ride the powerful tide of anti-Bush sentiment among progressives. It also buttressed his claims to be a Washington outsider, the most authentic agent of political change in the race. This appealed to independents.

So it’s little wonder that Obama takes his pledge to end the Iraq war very seriously. He undoubtedly regards it as a matter of keeping faith with his core supporters. At the same time, he was careful not to inflame old passions over the war. On the contrary, he rightly praised U.S. troops for their skill and valor, offered a graceful salute to his predecessor, and urged the country to move on.

In this respect, the speech was probably the most genuinely “post partisan” of his presidency. But it also raised questions about what Obama really thinks about the war.  He noted that U.S. troops, at tremendous sacrifice, toppled one of the world’s worst tyrants and gave Iraq a chance to embrace “a different destiny.” Does that mean he disagrees with the New York Times’ characterization of Iraq as a “tragic, pointless war”? Obama sounded ambiguous on the question of whether it was all worth it, but such reticence probably comes with the job of being President.

Whether the public will regard his declaration as an important milestone is another matter. Violence in Iraq is already down, thanks at least in part to the surge that Obama initially opposed but has since implicitly endorsed by putting the same general, David Petraeus, in charge of a similar escalation in Afghanistan. What’s more, 50,000 U.S. troops will remain in Iraq for the next 16 months, and at least some of them will be fighting al Qaeda insurgents. Truth to tell, the President did little more last night that endorse the timetable set forth in the Status of Forces Agreement (SOFA) the Bush administration negotiated with the Iraqi government.

For Obama, the significance of this moment is that it marks the transition to Iraqi responsibility for security. That’s fine, but America can’t simply wash its hands and walk away at the end of next year. Iraq didn’t ask to be invaded, or to be plunged into the hellish sectarian violence that followed. The United States has incurred an unavoidable moral obligation to help a decent political order emerge in Iraq. If that requires revisiting the SOFA, the administration shouldn’t be inflexible on the point.

In stressing the limits of America’s responsibilities, the President also drew parallels between Iraq and Afghanistan. The United States should stay in Afghanistan only as long as it takes to build the capacity of the Afghan government and security forces to defend the country against a vicious Taliban insurgency.

Obama, in fact, seemed to be implicitly advancing a new doctrine of limited U.S. military intervention. The unstated assumption: America probably will be forced to intervene again in failing and fragile states beset by terrorism or communal conflict. But we should make no open-ended commitments to counterinsurgency and national building. But war is seldom so tidy. The United States still has troops in South Korea, 57 years after the war there ended.

In all, it was an often confusing and even contradictory speech, as Fred Kaplan captured well today. It reflected the deep ambivalence of a man who rose to prominence on the strength of his anti-war stance, and now finds himself, as Commander in Chief, responsible for bringing no less than three wars – Iraq, Afghanistan and the fight against al Qaeda – to a successful conclusion.

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