When President Obama went to Oslo last week, he knew he was bringing with him a major contradiction. He was there to accept the Nobel Peace Prize at a time he was carrying out his responsibilities as a war president waging armed combat in two foreign countries.

He also knew the prize was being bestowed on him more out of promise than for performance, to the surprise and dismay of many committed pacifists around the world. As he had done on first hearing of the award, he acknowledged that "my accomplishments are slight" and that there were many others "far more deserving if this honor than I."

But rather than ignore the contradiction, he seized upon it to make a measured and rational defense for his decision to send 30,000 more Americans to Afghanistan, straightforwardly observing that "some will kill and some will be killed." The opening of his acceptance speech was hardly what the assembled pounders of swords into plowshares had anticipated.

In a sober and scholarly exposition of the concept of "a just war," Obama deftly focused in his remarks on Afghanistan, noting the war's origin as self-defense in response to the attacks of 9/ll planned there by terrorists harbored within its borders.

"The world rallied around America after the 9/11 attacks, and continues to support our efforts in Afghanistan because of the horror of those senseless attacks and the recognized principle of self-defense," he said. He cited that principle again, saying, "likewise, the world recognized the need to confront Saddam Hussein when he invaded Kuwait — a consensus that sent a clear message to all about the cost of aggression."

Obama used World War II against Hitler as a prime example of a just war, though the immediate trigger for American entry was self-defense against the Japanese attack on Pearl Harbor. Notably, he said nothing of the second war against Iraq, which was launched by his predecessor in violation of the U.N. Charter that declares self-defense the only justification for such action.

The president was, to be sure, on record long before in describing that Iraq adventure as "a dumb war," and his lengthy monologue in Oslo on just wars was an unspecified refutation of it. So were his observations, applauded by the audience, that "where force is necessary, we have a moral and strategic interest in binding ourselves to certain rules of conduct."

The United States, he said, "must remain a standard bearer in the conduct of war. ... That is why I prohibited torture. That is why I ordered the prison at Guantanamo Bay closed" — though that has not yet been accomplished. "And that is why I have affirmed America's commitment to abide by the Geneva Conventions," disavowed in the previous administration.

These observations were clear refutations of his predecessor as well, and he drew more applause in saying: "We lose ourselves when we compromise the very ideals we fight to defend. And we honor those ideals by upholding them not when it's easy, but when it is hard."

In a large sense, Obama's speech in Oslo was not only a defense of his own responsibilities and actions as a war president and of the whole concept of just wars, but also a deft reminder that he was indeed bringing a foreign policy change his pacifist audience could believe in. And that notion clearly was inherent in the Nobel judges' decision to bestow the prize on him.

Indeed, Obama seemed to go out of his way to argue that, as a wartime president, he is no George W. Bush. "America, in fact no nation, can insist that others follow the rules of the road if we refuse to follow them ourselves," he said. "For when we don't, our actions appear arbitrary and undercut the legitimacy of future interventions, no matter how justified. And this becomes particularly important when the purpose of military action extends beyond self-defense or the defense of one nation against an aggressor."

In this carefully crafted defense of the contradiction of his accepting the peace prize while escalating the war in Afghanistan, Obama managed in the process to make the case that the prize judges were justified in their decision after all.