Libertarians are opposed to war. They see war as a terrifying and self-serving exercise of state power. As Murray Rothbard put it in 1994: war is “an instrument for the aggrandizement of State power over the health, the lives, and the prosperity, of their subject citizens and social institutions.” Rothbard was not a pacifist, however. He thought that “a just war exists when a people tries to ward off the threat of coercive domination by another people, or to overthrow an already-existing domination.” Applying this standard, he concluded that the only just wars fought by the United States were the War of Independence and (ready for this?) the War for Southern Independence! All of the other wars (including World War II) were, for him, unjust wars.
I think that almost any view (pacifism, just war theory) is preferable to Rothbard’s view. This not just because, by reflective equilibrium, any conception of just war under which the South was morally justified in 1861 must fail. It is also because the collectivist ingredient in Rothbard’s definition is inconsistent with sound libertarian principles. To him, a just cause to fight is to end unjust domination of peoples. This violates a central tenet of libertarian thinking: normative individualism. Maybe no war is ever justified, but if anyone is, it has to be a war to defend persons, not peoples (whatever that means.)
So can a libertarian have a theory of just war? Let us assume that for the libertarian the initiation of force is morally wrong. It follows that defensive force may be right, at least sometimes. One may therefore say that the libertarian may approve of wars in self-defense, where citizens of the state are victims of a foreign attack. We need not assume here that those citizens are defending the state: because the foreign attack is directed at them, they are is defending themselves against an initiation of violence by foreigners.
However, this would be too quick, because in any war innocents die. They did not initiate violence against us, yet in response to the foreign attack we will be bringing about their deaths. Maybe the morally right thing to do is to surrender to the aggressor, if doing so would prevent us from causing the deaths of innocents. The libertarian who thinks that we cannot permissibly fight even defensive wars is a contingent pacifist. To him, if we could repeal the attack avoiding simultaneously the deaths of bystanders, then we could permissibly do it. But because we cannot avoid those deaths, we may not react against the attack: we must wave the white flag. This reasoning applies to the defense of others as well, because that action, too, will bring about the deaths of innocents. On this view, NATO’s intervention in Libya is wrong, not because it protects persons attacked by Khadaffy, but because it impermissibly kills innocents.
One way around this problem is to invoke the doctrine of double effect: sometimes bringing about the incidental deaths of innocents is permissible during a justified use of force against an aggressor. But can a libertarian endorse the doctrine of double effect? If we say no, we have to be prepared for the consequences: no war or revolution is ever justified. The Allies’ fight in World War II was unjustified, the uprising of the Libyans against Khadaffy is unjustified, and so was the American War of Independence. In World War II, I don’t mean only the entry of the United States in the war: fighting was impermissible also for the Poles, the French, the British, etc. For myself, I can’t stomach this. Perhaps a libertarian can say this: war is never justified except as defense of persons, and even then, considerations of proportionality (the death and destruction that wars, even initially justified ones, cause) turn war impermissible in most cases (certainly in more cases than, say, the various U.S. administrations thought.)
Libertarians, then, have a dilemma. If the libertarian is committed to this contingent form of pacifism (war, even defensive war, is always wrong because it brings about the deaths of innocents), then no war or revolution can ever be permissibly fought. If, on the other hand, the libertarian concedes that war is sometimes justified (when it is defensive) and accepts (some version of) the doctrine of double effect, then Rothbard’s warnings apply: we know from experience that the government will abuse this permission for all kinds of self-serving reasons.