Rob Alspaugh recently noted the importance of recognizing the relation between Aquinas's comments on war and his comments on strife or quarrelsomeness (rixa), so I thought I'd put up a translation of ST 2-2.41.1, which (like Rob) I think is more important than it is generally regarded. This is all quite quick & rough, and no doubt needs polish. The Latin is here; the Dominican Fathers translation is here.
It seems that quarrelsomeness is not always a sin. For quarrelsomeness is seen to be a kind of controversion, so that Isidore says, in the book of Etymologies, that the quarrelsome one (rixosus) is called so from the maw (rictu) of a dog, because he is always ready to contradict, is delighted by abusiveness, and provokes controversy. But controversy is not always a sin; therefore neither is quarrelsomeness.
Further, Genesis 26 says that the servants of Isaac dug another well, and and for that they also quarreled. But it is not to be believed that the household of Isaac quarreled in public and were not contradicted by him, if it were a sin. Therefore quarrelsomeness is not a sin.
Further, quarrelsomeness is seen to be a kind of partial war. But war is not always a sin; therefore quarrelsomeness is not always a sin.
But on the contrary is the fact that in Galatians 5 quarrels are put among the works of the flesh, which are such that those who do them shall not attain to the kingdom of God. Therefore quarrels are not only sins, they are mortal sins.
I reply that it must be said that controversy means a sort of contradiction in words, just as quarrelsomeness means a sort of contradiction in deeds; thus on Galatians 5 the Gloss says that quarrels are when out of wrath one strikes another. Thus quarrelsomeness is seen to be a kind of private war, which is enacted between private persons, not from some public authority, but rather from disordered will. Therefore quarrelsomeness always means a sin. And in him who unjustly attacks another it is a mortal sin, for it is not apart from mortal sin when one inflicts noxiousness on another even by works of the hand. And in him who defends himself from it, it can be without sin, or sometimes with venial sin, and also sometimes with mortal sin, according to the different impulses of his spirit, and the different ways of defending himself. Therefore if he does it in the spirit of repulsing harm, and defends himself with due moderation, it is not a sin, nor is it properly called quarrelsomeness on his part; if instead in the spirit of vindictiveness and hatred, or defending himself in excess of due moderation, it is always a sin, but a venial one if some light movement of hatred or vindictiveness is mingled with it, or because it does not greatly exceed moderate defense; but it is mortal when in a locked-in inflexible spirit he rises against his assailant to kill him or seriously hurt him.
To the first therefore it must be said that quarrelsomeness does not simply name controversy, but three things are put forward in the preceding passage by Isidore that show the disorder of quarrels. First, swiftness of spirit to controversion, which is signified when it is said, 'always ready to contradict', that is, whether the other speaks or does well or ill. Second, that there is in him delight in contradiction, so that there follows 'is delighted by abusiveness'. Third, that he provokes others to contradiction, so that there follows, 'and provokes controversy.'
To the second it must be said that it is not there understood that the servants of Isaac quarreled, but that the inhabitants of the land quarreled with them. Therefore those sinned, not the servants of Isaac, who endured the calumny.
To the third it must be said that for a war to be just, it is necessary that it be done originating from public authority, as said above. But quarrelsomeness is from a private feeling of wrath or hatred. If therefore the servant of the prince or judge by public authority attacks some who defend themselves, he is not said to be quarrelsome, but thgose who resist public authority, so then the one who attacks does not quarrel nor sin, but those who in a disordered way defend themselves.