Why are we bombing Libya? To prevent a massacre that would have “stained the conscience of the world,” President Obama proclaimed in his address to the nation last week.

“As president, I refused to wait for the images of slaughter and mass graves before taking action,” he said.

Say what you will about the Bush Doctrine of preventive war, at least it was directed at purported threats to American national security. Obama, by contrast, reserves the right to shoot first before the global conscience gets stained. In this vision, the U.S. military serves as a sort of Scotchgard for the World-Soul.

The president allowed that “America cannot use our military whenever repression occurs.” But when our interests, values, “unique abilities” and the will of the international community properly align — he’ll let us know when that is — we’ll act “on behalf of what’s right.”

But when it comes to armed humanitarianism, deciding what’s right may not be quite so simple, argues international relations scholar Alan J. Kuperman, author of The Limits of Humanitarian Intervention. First, generally speaking, “killers are quicker than intervenors”; second, “more intervention might actually lead to a net increase in killing,” not just from “collateral damage,” but by changing the incentives of actors on the ground.

Take the case of Rwanda. Our failure to intervene there in 1994 is now widely considered a shameful missed opportunity to avert mass murder.

But “had the United States tried to stop the Rwandan genocide,” Kuperman writes, “it would have required about six weeks to deploy a task force of 15,000 personnel and their equipment,” meaning that “by the time Western governments learned of the Rwandan genocide and deployed an intervention force, the vast majority of the ultimate Tutsi victims would already have been killed.”

Indeed, sometimes intervention increases the pace of violence. Serbian forces sped up ethnic cleansing in Kosovo in response to NATO’s March 1999 decision to bomb. “Most of their cleansing occurred in the first two weeks, and they managed to force out 850,000 Albanians.”

Further complicating matters is what Kuperman calls “the moral hazard of humanitarian intervention.” Such interventions can have the perverse effect of encouraging risk-seeking behavior by those expecting rescue.

That happened in the 1990s, Kuperman argues, when the policy of humanitarian intervention “convinced some groups that the international community would intervene to protect them from retaliation, thereby encouraging armed rebellions.”

But in the case of Bosnia-Herzegovina’s Muslims, for example, intervention occurred three years after secession, by which time an estimated 100,000 had been killed.

Last month, Jackson Diehl chided his Washington Post colleague George Will for asking, “Would not U.S. intervention in Libya encourage other restive peoples to expect U.S. military assistance?”

“Perhaps it would,” Diehl replied, “and if a powerful opposition movement appeared in Syria, and asked the West for weapons or air support to finish off the Assad regime, would that be a disaster?”

I don’t know, perhaps it would. Diehl, an ardent supporter of the Iraq War, ought to at least entertain the possibility. After all, that war turned out to be far more costly than folks like Diehl imagined at the outset. More than 4,000 Americans and anywhere from 40,000 to 100,000 Iraqi civilians have vanished so far in the fog of humanitarian war.

Would Syria present similar risks? Does Libya? Probably not, but one thing is certain: War is a bloody, uncertain business. When you decide to wage one, you need a good reason. Airy doctrines about impending moral stains on the international conscience don’t make the grade.

Here, a little less hubris might be more humanitarian.