On Thursday night, President Donald Trump confirmed that he had ordered a military strike against Iran, and then called it off, after a U.S. military drone was shot down by Iran earlier in the week.
Trump said he cancelled the military strike because the expected 150 Iranian casualties were not “proportional” to the destruction of a U.S. drone.
“Ten minutes before the strike I stopped it, [it was] not proportionate to shooting down an unmanned drone," Trump said.
The concept of proportionality in military conflict is rooted in what is often called the “just war theory,” most famously expounded by St. Thomas Aquinas.
Modern conflicts, which often involve missile and air strikes rather than pitched battles between troops, present a more complicated concept of war than in previous centuries but, theologians told CNA, just war theory remains applicable to modern warfare.
“The Catechism of the Catholic Church does a nice job of summarizing the criteria for entering into the use of military force for self-defense,” Miller told CNA, “though I tend to think of just war as more of a ‘doctrine’ than a ‘theory’ in the Church.”
Miller said the Church’s moral criteria are divided into two categories – the ius ad bellum and the ius in bellum, covering the criteria for resorting to war and how it is to be conducted once begun.
“Regarding proportionality, the Catechism says that ‘the use of arms must not produce evils or disorders graver than the evil to be eliminated.’ That would be the criterion that [President] Trump seems to be alluding to in saying what he did.”
While the prospect of air strikes causing casualties in response to shooting down an unmanned drone presents a clear case for weighing the proportionality of any response, Miller said, there can be some broader confusion about what proportionality means.
“It does not necessarily mean you cannot take the lives of more enemy combatants than have been lost on your side, that would be almost to say there was an obligation to lose the war. Nor does it necessarily mean that you cannot prosecute a war of self-defense in response to an initial strike – take for example the attack on Pearl Harbor, which was not necessarily intended to precede an invasion of the United States but nevertheless triggered a just response of war with Japan.”
Considering the specific example offered by Trump’s comments, Miller said that, at least based on what has been reported, the reasoning was less straightforward.
“If you think about the act of shooting down a drone – is that intended to be the beginning of a military action leading to further deadly damage to either American or allied personnel or interests? It might be clear that such an act was intended as a provocation, but not necessarily to war.”
Dr. Taylor Patrick O’Neill, assistant professor of theology at Mount Mercy University, told CNA that the ongoing nature of a threat was an important part of invoking a just military response.
“The first criterion for the use of military force is, of course, a just cause,” O’Neill told CNA.
“I think that in response to an act which has not yet caused any casualties, which is not part of an ongoing pattern of military aggression with lives at stake, there is a real question here about the just cause and the proportionality of engaging in a response which would certainly take lives – maybe many lives and even civilian ones at that.”
Miller agreed that responding with deadly force to a non-deadly provocation requires serious scrutiny.
“If it really is the case that the response to, say, the shooting down of an unmanned drone, is only intended to take out the infrastructure which made that possible and to prevent it happening again, it is all the more clear that proportionality really does come to bear when you are looking at taking human life – especially if those lives might include civilians. It becomes very problematic,” Miller said.
Modern conflicts often involve remote means of warfare and targets which are of unclear military status, such as governmental intelligence posts, radar stations, or other logistical installations. While the personnel in them might be primarily military, the presence of civilians has to weighed carefully in discerning military action.
“The classification of people involved can be very difficult to discern in modern conflicts,” O’Neill said.
“We don’t necessarily see artillery shelling enemy lines. With strikes from distance on military targets, there are people involved who might not be military personnel: they might be government intelligence workers or people in a grey area, but then there’s the possibility of just the civilian janitor in the building, how do you put them in the balance of proportionality? It makes things very difficult.”
O’Neill said that with modern means of warfare, there is a very high burden on governments to take all measures possible to limit the loss of potentially innocent human life.
“To have the moral justification and to make some calculus of proportionality, you have to have some good intelligence about who could be harmed – obviously there can be unintended consequences but you have to have a good amount of information about what the effects of a military action could be before you can judge if it is a just response.”
Miller emphasized the same point, telling CNA that even in response to the deaths of soldiers, any military response has to involve a difficult prudential judgment about the risk to civilian life.
“If lives are being lost and there is, say, an installation which is helping make that happen, a responsive attack there could be justified and proportionality satisfied, but only as long as everything that reasonably can be done to limit civilian casualties is done,” said Miller.
“Of course, so much of this is about thinking five or ten steps down the road, and it is about balancing the need to prevent an escalation while keeping an eye on all the possible unforeseen consequences,” O’Neill said.
“Whenever an action could have a double effect, proportionality becomes important,” Miller agreed.
“In war especially, but in moral thinking more broadly, where there is that risk, there is a prudential judgment to be made. Each situation needs to be assessed on its own merits and it is not always perfectly quantifiable, even almost a case of ‘you know it when you see it.’ There is no algorithm or mathematical formula for this.”