– This passage is taken from the Handbook of Moral Theology by Dominic M. Prummer O.P. –
280. Definition. War is an armed conflict between two opposing armies.It therefore differs from a duel, a quarrel, or an insurrection. War is either offensive when it is fought to obtain satisfaction for injury, or defensive when it is intended as a means of warding off unjust aggression from another ruler or State. Sometimes it is far from easy to distinguish an offensive war from a defensive war, since it does not always follow that the army which opens the war is conducting an offensive war.
Principle. A supreme authority, a just cause, and a right intention are required to justify the declaration of war. To wage war legitimately all the statutes of International law must be observed.
The first condition required for the declaration of war is self-evident. The second condition of war – a just cause – is best explained in the words of Francis de Vittoria: “There is only one just cause for entering upon war – violated rights.” Therefore one would not be justified in waging war for the purpose of self-aggrandizement or winning renown or in order to convert the pagan.
The third condition is also immediately evident.
Once war has broken out it is necessary to observe the statutes of International law of which the most important is: war is not waged against individuals but against an entire nation as a public person. Generally speaking, one is permitted to use everything necessary for crushing the resistance of the enemy. Soldiers commit grave sin if in the course of a just war they desert or cross over to the enemy lines.
– Lucas G. Westman