The most noteworthy thing about US President Barack Obama's Nobel lecture on December 9 in Oslo was its tragic sensibility. By tragic I mean his forthright acknowledgment of the reality of moral conflicts in matters of war and peace - situations in which even a course of action that is "just" or "necessary" or "lawful" has unavoidably bad consequences that we must keep in mind even when we pursue that course.The speech thus made the case for recognizing that using military force is sometimes the right thing to do - in self-defense, in defense of another nation against an aggressor, to prevent genocide, to stop a civil war that is systemically destabilizing - without suggesting that moral justification or legal authorization or multilateral support makes it bloodless or easily implemented or glorious.This might even be seen as a hard-earned advance in wisdom over the famous line in his 2002 Iraq speech that he is not opposed to all wars, just "dumb wars." One of his most profound messages in Oslo was that while some wars are worth fighting, at a fundamental level war is always "dumb" - or as he put it there, war is "an expression of human folly," "promises human tragedy" and (quoting Martin Luther King) "solves no social problem."This is an essential corrective to views held at many points on the political spectrum. It reminds us that whatever justification one finds most compelling, whether particularist vital national interests or universalist humanitarian concerns or both, and however sophisticated our approach, whether exploiting a "revolution in military affairs" or conducting "graduate-level counterinsurgency" or using multilateral "smart power," war and intervention remains a blunt force, which we should therefore rely on only to achieve - if we are very good and very lucky - blunt objectives.THIS IS a theme with a long history, but it is always difficult to keep clearly in mind because of its unsettling ambiguity and the way it complicates policy views that are felt as imperatives. This has been particularly true for idealists since the end of the Cold War removed what had been an uncomfortable constraint on our long-standing impulse to help disadvantaged peoples and share our virtues, and particularly true for realists since 9/11 underscored the threats to our immediate physical security, with both positions often converging on the need to "fix" failed states.And this is precisely why it is so important to be reminded by President Obama of both the necessity for, and the moral complexities inherent in, what he at one point referred to as "responsibility" - no doubt deliberately echoing Max Weber's discussion of the "ethics of responsibility" in his 1918 lecture "Politics as a Vocation": "He who lets himself in for politics, that is, for power and force as means, contracts with diabolical powers and for his action it is not true that good can follow only from good and evil only from evil, but that often the opposite is true."On this point, the most insightful work by Michael Walzer is not Just and Unjust Wars but his 1973 essay "Political Action: The Problem of Dirty Hands," in which he discusses Machiavelli, Weber and Albert Camus as examples of three subtly different traditions of understanding why even a just wartime leader would have "dirty hands" (as put in Sartre's play of that name): "Here is the moral politician: It is by his dirty hands that we know him." AND SO Obama's Nobel remarks help us see how fine a line we must walk in matters of peace and war.On the one hand, his speech concluded ringingly against "accept[ing] despair as the final response to the ambiguities of history" (quoting King), and in favor of ever striving for progress in justice, dignity and peace - even, where necessary, by force.On the other hand, his case for the utility of force was triply constrained - first, by the limited class of cases in which it is justified (vs. the much broader set of circumstances in which affirmative and negative measures other than war such as development assistance and sanctions are appropriate); second, by the importance of doing everything we can to adhere to moral standards in the use of force; and third, by his reminders that war is inherently unpredictable, that "we are fallible" and that "even those of us with the best of intentions will at times fail to right the wrongs before us."Given the long but often publicly obscured history of this idea of the tragic ambiguity of power, it is fitting to close with a poetic forerunner of Obama's fundamental argument.The following lines are from an early speech in Henry V in which the king is probing the arguments of one of his lords in favor of war with France:My learned lord, we pray you to proceed And justly and religiously unfold Why the law... should, or should not, bar us in our claim: God forbid, my dear and faithful lord, That you should fashion, wrest, or bow your reading, Or nicely charge your understanding soul With opening titles miscreate, whose right Suits not in native colors with the truth; For God doth know how many now in health Shall drop their blood in approbation Of what your reverence shall incite us to.Therefore take heed how you impawn our person, How you awake our sleeping sword of war; We charge you, in the name of God, take heed; For never two such kingdoms did contend Without much fall of blood; whose guiltless drops Are every one a woe, a sore complaint 'Gainst him whose wrong gives edge unto the swords That make such waste in brief mortality.The writer is a Jennings Randolph Guest Scholar at the United States Institute of Peace. From November 2008 to February 2009, he served as one of five co-directors of a 200-person interagency strategic assessment of the Middle East organized by US Central Command, and helped lead the assessment mission to Afghanistan.