How should a morally conscientious person decide whether to enlist in the armed forces? Jeff McMahan argues that, by enlisting, you increase the probability that you will engage in wrongful killing in unjust wars. But this does not settle the matter. Many activities—learning to drive, say—increase the risks that we will act wrongfully in the future; that alone cannot render the activity impermissible. We need to know not only the risk of wrongdoing, but also the expectation of good. Whether it is permissible to volunteer depends on whether your expectation of doing good is greater than your expectation of doing ill.

Working out these expectations is challenging. But here is one helpful approach. First, ask whether the institution is justified. This can mean two things: Is it morally permissible to have these armed forces, compared with having none at all? And are our armed forces the best they can be, morally, given feasible alternative institutions? Call the first minimal justification, and the second full justification. One necessary condition of both is that the expectation of good is greater than the expectation of wrong. If our armed forces are expected to do more wrong than good, relative to having no such institutions, then they cannot be minimally justified; if we expect them to do more wrong than good, relative to feasible alternatives, then they cannot be fully justified.

Justification relates to the permissibility of participation. Institutions are not reducible to the operatives who fill their roles. But obviously those operatives are significant. Now suppose the armed forces were fully justified. Surely participation by at least some of their operatives must therefore be permissible. Perhaps some could be responsible for the lion’s share of the good, and others responsible for the bulk of the bad, so that participation by the first group is permissible, but not the second group. But this is unlikely. In warfare those likely to do most good are also those likely to do most wrong.

So, if the institution is fully justified, then the participation necessary for the institution to function should be permissible, at the individual level. If the armed forces are fully justified, then volunteers are permitted to accept the risks of wrongdoing that come with joining up.

If having armed forces is justified, some people must take moral risks. 

What if the armed forces are minimally justified? There are two possibilities. First, we use the same reasoning as for full justification: the institution cannot be minimally justified if its operatives are not permitted to perform the roles the institution depends on for its functioning. So arguing that volunteering is morally impermissible would amount to arguing that the armed forces are not minimally justified.

Second, perhaps minimally justified armed forces include roles that are necessary for their functioning but are impermissible for those who take them up because their expectation of wrong is greater than their expectation of good. To have these justified institutions, some people must take moral risks that one ought not take. If minimally justified institutions require that some operatives get dirty hands, then all of those able should have an equal prospect of bearing those moral risks: we should endorse conscription. But suppose that some among us volunteer to take on that burden. How should we evaluate their decision? Barring wrongful motivations, and if the decision is genuinely voluntary, we should celebrate it as a remarkable display of not only physical but also moral courage. They are putting both their lives and their souls at risk so others don’t have to, to sustain a morally justified institution from which we all benefit.

So if the armed forces are either minimally or fully justified, then volunteers are either acting permissibly because the expectation of wrongdoing is outweighed by the expectation of good, or acting heroically because they are running a moral risk in order to maintain minimally justified institutions.

The real question, then, is not whether individuals are permitted to join the armed forces, but whether the armed forces themselves are at least minimally justified. I think the U.S. military is minimally justified; I suspect McMahan would agree. Of course, moral improvements are feasible; perhaps selective conscientious objection would be among them. But I am not sure. I’ll leave the military experts to speak to its feasibility, but note this: McMahan concedes that malingering will occur only in wars where the combatants’ own interests aren’t directly at stake. Much of the world’s population depends for its security not on their own militaries but on that of the United States. And given American power, no state is likely to directly threaten the U.S. mainland. If selective conscientious objection would lead to malingering when the United States is not directly threatened, then it would radically undermine the security of the world beyond American borders.