Listening to U.S. President Barack Obama's speech in Brussels this week, I found myself thinking, "He's got his voice back." This thought came right around the moment when he deployed the expression "we believe" as a rhetorical device to underline the universality of faith in free expression and free markets and in "an international system that protects the rights of both nations and people." Obama is a belief-driven leader who in recent months has had very few opportunities to project his beliefs upon the world. Now, suddenly, he has a cause.

I wonder whether Russian President Vladimir Putin, whose strategic talents have been so favorably compared to Obama's, has not in fact given the American president an immense gift. Leaders need obstacles; better still, they need enemies. President Bill Clinton, ruling at the noontide of American power, never had an adversary against whom to show his mettle. The George Bush who came before him was fortunate to have Saddam Hussein, and the Bush who came later had Osama bin Laden. You could hardly do better. Of course, the difference between the two Bushes shows that a president can use that morally charged confrontation to unite the nation and the world, or to divide them.

The second George Bush discredited American moralism by reducing it to the cowboy slogan of "Either you're with us or against us." As a presidential candidate, Obama found his footing by declaring, in a campaign debate with Hillary Clinton, that, unlike either Bush or then-Senator Clinton, he would talk to any American rival without preconditions. Afterward, Samantha Power, then one of Obama's chief foreign-policy advisors, confided to me that he had found this unplanned exchange "orienting." That's who he was -- the dispassionate statesman who would dispense with moral posturing in order to find shared interests.

Thus was born "engagement," the dominant foreign-policy paradigm of Obama's first years in office. Over time, however, Obama discovered the limitations of finding common ground. The Iranian leadership rejected his overtures; only the ever-tightening vise of sanctions has brought Tehran to negotiate over its program of nuclear enrichment. In the Arab world, engagement foundered on its own contradictions because Obama had to choose between engaging with regimes and engaging with citizens who despised those regimes. The "reset" with Russia, which bore fruit in Obama's first two years, had flagged long before Putin unleashed the hounds in Crimea.

Engagement ran its course. What was worse was that the Arab Spring, once a source of transcendent hope, ultimately entangled Obama in portentous conflicts with no morally satisfying solution. On Syria, the White House convinced itself that it would do more harm than good by seriously supporting the insurgents, yet by withholding that support helped give birth to a Hobbesian setting that really does seem beyond rescue. Egypt, though less monstrous, is just as bewildering, for the administration supported a democratically elected Islamist government that the Egyptian people themselves turned against en masse. The same masses who bled and died to overthrow a military dictator have now embraced a new one. Against what, and with whom, is America to stand? The "pivot to Asia," the world's most overadvertised foreign-policy venture, seemed designed to leave behind this torrid and tormented zone for the cool uplands of sovereign states bent on increasing their GDP (and fending off China's rising ambitions).

Then Putin yanked the administration back to the world of aggression. Putin, it's true, has dictated the terms of the contest and made the United States and its European allies look reactive and improvisatory. Republicans are having a field day with Obama's "weakness." If Putin invades eastern Ukraine, the Cold War bitter-enders will feel thoroughly vindicated. Nevertheless, I suspect that neither Putin's gloating nor Sen. John McCain's mockery will last. Putin is the foil Obama has been waiting for.

The annexation of Crimea, as Obama declared, violated the rights of "both nations and people," thus outraging the sovereignty-minded nations of Asia as well as the liberal democracies. Putin sought to roll back history by redrawing settled borders. He sought to stifle a mass democratic uprising in Ukraine. He bullied and he lied. (Really, he's almost as satisfying a villain as Saddam.)

Perhaps this will serve as Obama's new "orienting" moment. Already, he has taken the lead in imposing sanctions on Russia, while orchestrating the global response to the annexation of Crimea. In his Brussels speech, he noted that "a coldhearted calculus" of interests in Ukraine might dictate that the United States "look the other way." Yes, that's a typical Obamian straw man -- on the order of "some say …" -- but the assertion also allowed him to reaffirm those "universal" principles that guide American foreign policy. Maybe it took Putin to remind the president of those principles. Obama has always said that he admires realists like former Secretary of State James Baker and former National Security Advisor Brent Scowcroft, but his sense of his own destiny, and of America's, is too exalted for him to accept the coldhearted calculus that made those men so effective.

What would it mean for the Putin challenge to reorient Obama? First, it might give Obama a large and urgent sense of purpose. Secretary of State John Kerry, consumed by the sense of urgency and hyperbolic self-assurance Obama once exuded, has taken on every impossible portfolio; meanwhile, the White House has complacently waved as he disappeared over the horizon. Protecting "the West" is a fine job for a president once accused of harboring "post-European" sentiments. Only the United States can organize the military, diplomatic, and economic response to Russia's provocation. Kerry will wind up doing that too, but there must be no doubt, including in Putin's mind, that Obama himself is driving the policy.

That response must be rhetorical as well. The theme that ran through Obama's Brussels speech is that Putin has thrown down a challenge to what had seemed to be a consensual worldview and thus has committed aggression against an idea as well as against a place. That is what makes Putin so excellent a villain for Obama, a man who has a gift for propounding large ideas in the public sphere. Putinism is a bad idea that must be countered by a good one. It's all too easy to concede, as for example India has, that Russia had "legitimate interests" in Ukraine. Obama needs to drive home the principle that no nation can define its interest in a way that permits it to violate international norms.

One thing Obama believes in deeply, as anyone who has read his national security strategy would recognize, is international law and institutions. Putin has quite helpfully reminded the world of why a rule-based international order matters. Obama has the opportunity to wrap himself in that flag. If he is going to do so, however, he will have to speak to the American people as well, since the United States both upholds and at times threatens that order. Thanks to Republicans in the House, for example, the United States is now the only major country that has refused to endorse reforms in the International Monetary Fund that will increase the authority of emerging countries; conservatives refuse to accept that U.S. voting rights in that institution should modestly shrink to reflect the growing power of other states.

Come to think of it, convincing the American people of the merits of a rule-based international order would be another noble task for Obama to set for himself during the final third of his tenure. He could start more modestly by convincing citizens that the world beyond "the homeland" is a place of opportunity as well as a threat. The United States does stand for something in the world. Perhaps it has taken Vladimir Putin to remind us of that.

Image: Ed Johnson / Foreign Policy