Just War Theory splits war into two separate moral questions

Philosophers in this tradition argue a war's justice depends on two independent tests: jus ad bellum asks whether there's a just cause, legitimate authority, and last resort for fighting at all, while jus in bello asks whether the fighting itself stays proportionate and spares civilians. A war can pass one test and fail the other — a righteous cause fought without restraint is still unjust. The tradition runs from Augustine of Hippo through Thomas Aquinas to modern international law.

— Wikipedia contributors, Just war theory — Tradition spanning Augustine of Hippo, Thomas Aquinas, Hugo Grotius, and modern international law