How should a democracy decide when to compromise its ideals in pursuit of victory?

The governing principle of international law is “proportionality,” a widely misunderstood concept. It does not mean that there must be roughly the same number of deaths on both sides of a conflict. It does mean that when an army selects a legitimate military target for attack, knowing that the attack may harm civilians, the number of anticipated casualties must be proportional to the military value of the target. For example, if a low-ranking soldier has taken refuge in a school, hospital, or mosque, and attacking him would risk hundreds of civilian lives, such an attack would be disproportional to the military value of killing the soldier — and would therefore constitute a war crime.

If, on the other hand, the target were Osama bin Laden, or the current leader of ISIS, the calculus might be different. Former President Clinton recently implied that he could have gotten bin Laden before the terrorist attacks of 9/11 were carried out, but at the risk of civilian casualties. If Clinton had known that killing bin Laden then would have saved the lives of 3,000 Americans on 9/11 — something that couldn’t have been known with certainty — he might well have ordered the attack, despite the risk to foreign civilians. These are the sorts of difficult moral calculations that are supposed to be made, often in the fog of war, by the rule of proportionality. President Obama or his successor may well have to make similar calculations as the United States and its allies seek to destroy ISIS.

Israel had to engage in a proportionality analysis when it sent ground troops to destroy more than 30 tunnels built by Hamas on the Gaza-Israel border. These “terror tunnels,” which were designed for attacks against Israeli civilians and soldiers, begin in Gaza and end on the Israeli side of the border, making them hard to locate. I was in one of them just days before the recent fighting began. The exit was close to a kindergarten with more than 50 children. Fortunately, a Bedouin tracker discovered an air hole in a field that led the Israeli Army to the tunnel. But dozens of other tunnels could not be found and disabled without the deployment of ground troops on the Gaza side of the border.