Objection 1. It would seem lawful for clerics and bishops to fight. For, as stated above (Article 1), wars are lawful and just in so far as they protect the poor and the entire common weal from suffering at the hands of the foe. Now this seems to be above all the duty of prelates, for Gregory says (Hom. in Ev. xiv): "The wolf comes upon the sheep, when any unjust and rapacious man oppresses those who are faithful and humble. But he who was thought to be the shepherd, and was not, leaveth the sheep, end flieth, for he fears lest the wolf hurt him, and dares not stand up against his injustice." Therefore it is lawful for prelates and clerics to fight.
Objection 2. Further, Pope Leo IV writes (xxiii, qu. 8, can. Igitur): "As untoward tidings had frequently come from the Saracen side, some said that the Saracens would come to the port of Rome secretly and covertly; for which reason we commanded our people to gather together, and ordered them to go down to the seashore." Therefore it is lawful for bishops to fight.
Objection 3. Further, apparently, it comes to the same whether a man does a thing himself, or consents to its being done by another, according to Romans 1:32: "They who do such things, are worthy of death, and not only they that do them, but they also that consent to them that do them." Now those, above all, seem to consent to a thing, who induce others to do it.
But it is lawful for bishops and clerics to induce others to fight: for it is written (xxiii, qu. 8, can. Hortatu) that Charles went to war with the Lombards at the instance and entreaty of Adrian, bishop of Rome. Therefore they also are allowed to fight.
Objection 4. Further, whatever is right and meritorious in itself, is lawful for prelates and clerics. Now it is sometimes right and meritorious to make war, for it is written (xxiii, qu. 8, can. Omni timore) that if "a man die for the true faith, or to save his country, or in defense of Christians, God will give him a heavenly reward." Therefore it is lawful for bishops and clerics to fight.
On the contrary, It was said to Peter as representing bishops and clerics (Matthew 16:52): "Put up again thy sword into the scabbard [Vulgate: 'its place'] ["Scabbard" is the reading in John 18:11."] Therefore it is not lawful for them to fight.
I answer that, Several things are requisite for the good of a human society: and a number of things are done better and quicker by a number of persons than by one, as the Philosopher observes (Polit. i, 1), while certain occupations are so inconsistent with one another, that they cannot be fittingly exercised at the same time; wherefore those who are deputed to important duties are forbidden to occupy themselves with things of small importance. Thus according to human laws, soldiers who are deputed to warlike pursuits are forbidden to engage in commerce [Cod. xii, 35, De Re Milit.].
Now warlike pursuits are altogether incompatible with the duties of a bishop and a cleric, for two reasons. The first reason is a general one, because, to wit, warlike pursuits are full of unrest, so that they hinder the mind very much from the contemplation of Divine things, the praise of God, and prayers for the people, which belong to the duties of a cleric. Wherefore just as commercial enterprises are forbidden to clerics, because they unsettle the mind too much, so too are warlike pursuits, according to 2 Timothy 2:4: "No man being a soldier to God, entangleth himself with secular business." The second reason is a special one, because, to wit, all the clerical Orders are directed to the ministry of the altar, on which the Passion of Christ is represented sacramentally, according to 1 Corinthians 11:26: "As often as you shall eat this bread, and drink the chalice, you shall show the death of the Lord, until He come." Wherefore it is unbecoming for them to slay or shed blood, and it is more fitting that they should be ready to shed their own blood for Christ, so as to imitate in deed what they portray in their ministry. For this reason it has been decreed that those who shed blood, even without sin, become irregular. Now no man who has a certain duty to perform, can lawfully do that which renders him unfit for that duty. Wherefore it is altogether unlawful for clerics to fight, because war is directed to the shedding of blood.
Reply to Objection 1. Prelates ought to withstand not only the wolf who brings spiritual death upon the flock, but also the pillager and the oppressor who work bodily harm; not, however, by having recourse themselves to material arms, but by means of spiritual weapons, according to the saying of the Apostle (2 Corinthians 10:4): "The weapons of our warfare are not carnal, but mighty through God." Such are salutary warnings, devout prayers, and, for those who are obstinate, the sentence of excommunication.
Reply to Objection 2. Prelates and clerics may, by the authority of their superiors, take part in wars, not indeed by taking up arms themselves, but by affording spiritual help to those who fight justly, by exhorting and absolving them, and by other like spiritual helps. Thus in the Old Testament (Joshua 6:4) the priests were commanded to sound the sacred trumpets in the battle. It was for this purpose that bishops or clerics were first allowed to go to the front: and it is an abuse of this permission, if any of them take up arms themselves.
Reply to Objection 3. As stated above (23, 4, ad 2) every power, art or virtue that regards the end, has to dispose that which is directed to the end. Now, among the faithful, carnal wars should be considered as having for their end the Divine spiritual good to which clerics are deputed. Wherefore it is the duty of clerics to dispose and counsel other men to engage in just wars. For they are forbidden to take up arms, not as though it were a sin, but because such an occupation is unbecoming their personality.
Reply to Objection 4. Although it is meritorious to wage a just war, nevertheless it is rendered unlawful for clerics, by reason of their being deputed to works more meritorious still. Thus the marriage act may be meritorious; and yet it becomes reprehensible in those who have vowed virginity, because they are bound to a yet greater good.
Source: Summa Theologica (New Advent)