Christopher Preble offers five criteria for determining whether the U.S. should intervene militarily overseas. This strikes me as the most important of the five:

Thus, the fifth and final rule concerning military intervention is force should be used only as a last resort, after we have exhausted other means for resolving a foreign policy challenge that threatens vital U.S. national security interests.

This would seem to be the most straightforward rule and the easiest to follow, but in practice many advocates for preventive wars and wars of choice ignore it while pretending to respect it. Hillary Clinton recently claimed that she believes in using force only as a last resort, but from her record we know that to be false. Bombing Libya in March 2011 was not the last means available to the U.S. and its allies to address the civil war and the danger it posed to civilian lives, nor was invading Iraq in 2003 the last remaining option in resolving outstanding questions about Iraq’s alleged weapons programs. It is obvious that it is not possible for preventive wars to be waged as a last resort, and their supporters believe that using force is appropriate at a much earlier stage. If we take the last resort standard seriously, it means that preventive war can never be an option because it is inherently too hasty and unjust.

I agree with all of Preble’s rules, but I would add another one to the list: the use of force must not be likely to produce worse evils than the ones that already exist. This is a very difficult bar to clear, but it has to be cleared if a military intervention isn’t going to make the other country (or countries) involved worse off than they were. One of the recurring problems in our debates over military intervention is that negative consequences of intervention for the affected country and its neighbors are often treated as an afterthought, or the potential negative consequences are dismissed by asserting that “it can’t be any worse than it is now.” The latter response was used frequently during the Iraq war debate to deny the possibility that Iraq could be worse off after regime change. We have heard much the same thing in other debates since then, and that is usually because interventionists either can’t imagine that U.S. military action can make a situation worse.

As Preble says, there should be “a built-in presumption against the use of force,” and the burden of proof in any debate over military action has to be squarely on the advocates of intervention. After all, interventionists are the ones insisting that the U.S. maim and kill people in some other part of the world, and that should never be treated as the default response to events overseas.