The idea that the United States had a moral obligation to intervene to save civilians from ruthless actions of their own government has played a prominent role in justifying the military intervention in Libya. In his televised address on Monday, President Obama stated that waiting any longer meant the city of Benghazi “could suffer a massacre that would have reverberated across the region and stained the conscience of the world.” Much has also been written about the pressing of this argument by certain members of Mr. Obama's administration—especially White House aide Samantha Power, who covered the Bosnia war as a journalist and had written also about the genocide in Rwanda.

One has to ask: who gets to define, seemingly on behalf of all Americans, which moral principles will govern national policy? That question is seldom posed; too often appeals to do what is morally right are accompanied by an apparent assumption that the direction in which morality points is clear. If the morally right course is not followed, according to this outlook, it is because morality lost out to realpolitik, stinginess, cowardice, or inattention. The outlook has not been confined to liberal interventionists; in the past it has been seen at least as much, for example, among the anti-war left.

But very often the direction in which morality points is not clear. The fact that morally infused liberal interventionists and anti-war activists who are just as morally infused may be on opposite sides of an issue demonstrates the lack of clarity. So does the way that moral concepts that get the most attention tend to grow out of personal experiences such as Power's. The more one gets away from emotional reactions based on such experiences and to the kind of rigorous analysis from first principles in which a moral philosopher might engage, the more the lack of an obviously right direction becomes apparent.

Consider ideas from one of the most prominent contemporary thinkers who has addressed the moral philosophy of war and peace, Michael Walzer. In his book Just and Unjust Wars, Walzer devotes a chapter to intervention by outside powers in internal wars. His main principle is that interventions are unjust if they prop up a party to the conflict that would not otherwise be able to sustain itself—which, according to proponents of the most recent intervention, was true of the Libyan rebels. Walzer provides an exception for humanitarian intervention, when it has a reasonable chance of success and responds to acts that—in words similar to the president's—“shock the moral conscience of mankind.” But he clearly has in mind situations that are more certain and ongoing than mere possibilities of what a ruler “could” do if left unopposed. He cites as an example the atrocities of a Punjabi army against Bengali civilians in 1971 in what was then East Pakistan. “Millions of Bengalis fled into India,” Walzer writes, “and their arrival, destitute, hungry, and with incredible stories to tell, established the moral foundation of the later Indian attack.” Consistent with what he laid out in his earlier book, Walzer has recently argued that the intervention in Libya is unjustified. “A military attack of the sort now in progress is defensible only in the most extreme cases,” he says. “Rwanda and Darfur, where we didn’t intervene, would have qualified. Libya doesn’t.”

The point is not that Walzer is necessarily right (I disagree with some aspects of his ethical system relating to warfare) and that President Obama and Samantha Power are necessarily wrong, but rather that there is no single option that is clearly the morally correct one. A decision such as the one taken on Libya is difficult and complex partly because of competing priorities, trade-offs between different interests, and empirical uncertainties. But even if those complications were shoved aside and one focused solely on treading the morally correct path, the decision still would be difficult and complex. It would be so partly because different ethical values (such as peace and justice) often collide, because our leaders have to weigh their responsibilities to their own citizens as well as responsibility to mankind in general, and because the consequences of decisions include not only the immediate impact but also secondary and tertiary effects, some of which also may involve human suffering or its avoidance.

Certainly we should seek the morally right course when confronted with a challenge like the conflict in Libya. But identifying that course should be a matter of public debate, not the product of any single perspective. It should not be left just to moral philosophers, nor should it be left to White House aides.