The principle is clearly very hard to argue against - as a principle. Those who are not killing others, regardless of their political views, nationality or location should be left alone. I have always regarded this as a “quid-pro-quo” sort of argument. You would not want members of your own society to be killed or injured in such a way and so you should not kill others. Kant would thoroughly approve.
But does this principle work in reality? Can a nation or group start a conflict and really hope to keep innocents out of it?
Well before you say no - and start to slag off St Thomas Aquinas - it might be worth remembering that warfare was very different in Thomas’ day. There was, quite literally, a field of battle; rulers agreed through negotiation where and when the battle was fought and codes were followed as a matter of chivalric honour. This was wonderfully satirised in Asterix in Britain (click on the image to enlarge)
With this in mind it is not surprising that Aquinas views the killing of innocent with moral horror. You would have had to go out of your way as a Knight of foot soldier to kill bystanders. You could be in the next village, out of ear-shot, and not even know a war was going on! However in the modern era, with weaponry that would have been unimaginably powerful to Thomas and totally indiscriminate when fired or detonated, the death of innocents has seemingly become an inevitability. Added to this war is not fought under the same strict conditions and, as can be seen in Israel/Palestine and The Ukraine at the moment, war is not necessarily even “declared” as such.
Has warfare changed so much that the JWT is now redundant?
On the one hand you could say; it is a principle, and the principle is a good one even if it is not often maintained in the modern world.
Or you might say; warfare is so powerful and indiscriminate that you cannot go to war “morally” in the modern world and so one should not go to war (the thinking of Contingent Pacifism)
Bellamy, Alex. Just Wars. Cambridge: Polity Press, 2006.
Orend, Brian. The Morality of War. Toronto: Broadview Press, 2006.