Don't underestimate the costs of inaction.

• Author: Teson, Fernando R.
• Position: Viewpoint essay

Current events in Syria and Iraq have rekindled talk about humanitarian intervention. The amply documented atrocities perpetrated by the Islamic State (ISIS) range from public beheading to rape, forced conversion, and expulsion. The United States and a few other countries are already attacking ISIS from the sky and giving some aid to resistors on the ground. But these bombings will not be sufficient to stop ISIS' crimes. By all appearances, only a full invasion with ground troops could get the job done. And Americans are weary of invasions.

Most libertarians oppose intervention on principle. But let us take a moment to focus not on principles but consequences. Arguments for or against intervention should always consider costs; the problem lies in calculating those costs. We know that every war kills and destroys. We also know that sometimes war produces a positive result. How do we measure and weigh the outcomes?

I do not have enough information to say with any confidence whether the costs of a full invasion to defeat ISIS would be acceptable. But I can propose two guidelines to help policy makers think through the problem.

First, when contemplating military action, leaders should consider the price of inaction as well. Second, the more distant effects--such as future unrest, wars, and massacres--must be evaluated alongside immediate results. These sound like simple things, but they are neglected surprisingly often in public debate over foreign policy.

Here's the tricky part: The effects of intervention, like those of any contemplated human action, have to be evaluated ex ante, that is, from the standpoint of the person who is considering whether to act beforehand, and not only ex post, that is, when all the effects are known after the fact. Especially when you consider that inaction, too, could have led to unforeseen miseries. In other words, hard though it may be to accept, a disastrous outcome is not itself proof that a decision to go to war was the wrong one.

People who defended the 2003 Iraq War (myself included) did not accurately predict all the bad things that the invasion would enable, including the prolonged insurgency and the continued inability of the Iraqi leadership to preserve the gains of Saddam Hussein's ouster. The stronger predictions about the short- and mid-term effects of the war came from noninterventionists, who correctly argued that the invasion would open a Pandora's box in the region. It might therefore be...