We must now consider war, under which head there are four points of inquiry:
(1) Whether some kind of war is lawful?
(2) Whether it is lawful for clerics to fight?
(3) Whether it is lawful for belligerents to lay ambushes?
(4) Whether it is lawful to fight on holy days? _______________________
FIRST ARTICLE [II-II, Q. 40, Art. 1]
Whether It Is Always Sinful to Wage War?
Objection 1: It would seem that it is always sinful to wage war.
Because punishment is not inflicted except for sin. Now those who
wage war are threatened by Our Lord with punishment, according to
Matt. 26:52: "All that take the sword shall perish with the sword."
Therefore all wars are unlawful.
Obj. 2: Further, whatever is contrary to a Divine precept is a sin. But war is contrary to a Divine precept, for it is written (Matt. 5:39): "But I say to you not to resist evil"; and (Rom. 12:19): "Not revenging yourselves, my dearly beloved, but give place unto wrath." Therefore war is always sinful.
Obj. 3: Further, nothing, except sin, is contrary to an act of virtue. But war is contrary to peace. Therefore war is always a sin.