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.
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"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."