In Just and Unjust Wars Michael Walzer gives some moral guidelines concerning war. But neither Walzer nor just war theory provides clean cut rules for military behavior. There is always a degree of uncertainty in warfare, and this uncertainty is centrally important to just war analysis, particularly the principle of proportionality.

Just war theory usually has two “in war” guidelines: discrimination and proportionality. Discrimination argues that military force ought only to be applied against combatants, and civilians should be exempt. There does not seem to be much uncertainty here. It is generally incorrect to kill people, and even in war, the logic goes, civilians should be spared since they pose no direct threat to combatants. Fair enough. But the waters become murkier when we make the jump to proportionality.

Proportionality argues that the use of force should be limited such that the achieved goal justifies the injury inflicted. This injury, Walzer tells us, may sometimes include civilians. Now at first this sounds utilitarian: if the object seems useful enough, proceed with your destruction. But in fact “proportionality” is a blend of principle and uncertainty and is not utilitarian. There are actually two principles at work: the first is to avoid destruction. However, since we are currently engaged in war it is assumed we believe there is some higher principle at stake—perhaps our territorial integrity or the protection of our population.

Thus there is a collision of principles: the principle to avoid human destruction versus, say, the right to territorial integrity. This is where uncertainty comes alive. Just war theory will say that the right of national territory is in fact the higher principle. But how “higher” is it? And what level of retaliation does it justify? Are we allowed, morally speaking, to lie to the enemy? Can we kill the enemy soldiers? Bomb his cities? Invade his country? And after we win, can we devastate his country to remove a future threat? The answer, to all of these questions, is: we’re not sure. There is no rule. So, we introduce proportionality, which in a reworded definition could read: “keep your application of violence at the lowest level possible that will still allow you to achieve your goal in fighting.” We are uncertain how much violence we can morally employ in response to their aggression, so let’s limit it all we can.

To borrow from Clausewitz, the “fog of war” encompasses more than strategic and tactical moves; it also includes the realm of moral decision-making. Since we are never certain what exigencies justify violence against human life, nor the level of that violence, we are well advised at all times to proceed cautiously. This is the principle of proportionality.