Found over on MercyJourney:
In the first place, we must distinguish between an occupation and the man who holds it, between a work and the man who does it. An occupation or a work can be good and right in itself and yet be bad and wrong if the man who does the work is evil or wrong or does not do his work properly. The occupation of a judge is a valuable divine office. This is true both of the office of the trial judge who declares the verdict and the executioner who carries out the sentence.2 But when the office is assumed by one to whom it has not been committed or when one who holds it rightly uses it to gain riches or popularity, then it is no longer right or good. The married state is also precious and godly, but there are many rascals and scoundrels in it. It is the same way with the profession or work of the soldier; in itself it is right and godly, but we must see to it that the persons who are in this profession and who do the work are the right kind of persons, that is, godly and upright, as we shall hear.
In the second place, I want you to understand that here I am not speaking about the righteousness that makes men good in the sight of God. Only faith in Jesus Christ can do that; and it is granted and given us by the grace of God alone, without any works or merits of our own, as I have written and taught so often and so much in other places. Rather, I am speaking here about external righteousness which is to be sought in offices and works. In other words, to put it plainly, I am dealing here with such questions as these: whether the Christian faith, by which we are accounted righteous before God, is compatible with being a soldier, going to war, stabbing and killing, robbing and burning, as military law requires us to do to our enemies in wartime. Is this work sinful or unjust? Should it give us a bad conscience before God? Must a Christian only do good and love, and kill no one, nor do anyone any harm? I say that this office or work, even though it is godly and right, can nevertheless become evil and unjust if the person engaged in it is evil and unjust.
In the third place, it is not my intention to explain here at length how the occupation and work of a soldier is in itself right and godly because I have written quite enough about that in my book Temporal Authority: To What Extent It Should Be Obeyed.3 Indeed, I might boast here that not since the time of the apostles have the temporal sword and temporal government been so clearly described or so highly praised as by me. Even my enemies must admit this, but the reward, honor, and thanks that I have earned by it are to have my doctrine called seditious and condemned as resistance to rulers.4 God be praised for that! For the very fact that the sword has been instituted by God to punish the evil, protect the good, and preserve peace [Rom. 13:1–4; I Pet. 2:13–14] is powerful and sufficient proof that war and killing along with all the things that accompany wartime and martial law have been instituted by God. What else is war but the punishment of wrong and evil? Why does anyone go to war, except because he desires peace and obedience?
Now slaying and robbing do not seem to be works of love. A simple man therefore does not think it is a Christian thing to do. In truth, however, even this is a work of love. For example, a good doctor sometimes finds so serious and terrible a sickness that he must amputate or destroy a hand, foot, ear, eye, to save the body. Looking at it from the point of view of the organ that he amputates, he appears to be a cruel and merciless man; but looking at it from the point of view of the body, which the doctor wants to save, he is a fine and true man and does a good and Christian work, as far as the work itself is concerned. In the same way, when I think of a soldier fulfilling his office by punishing the wicked, killing the wicked, and creating so much misery, it seems an un-Christian work completely contrary to Christian love. But when I think of how it protects the good and keeps and preserves wife and child, house and farm, property, and honor and peace, then I see how precious and godly this work is; and I observe that it amputates a leg or a hand, so that the whole body may not perish. For if the sword were not on guard to preserve peace, everything in the world would be ruined because of lack of peace. Therefore, such a war is only a very brief lack of peace that prevents an everlasting and immeasurable lack of peace, a small misfortune that prevents a great misfortune.
What men write about war, saying that it is a great plague, is all true. But they should also consider how great the plague is that war prevents. If people were good and wanted to keep peace, war would be the greatest plague on earth. But what are you going to do about the fact that people will not keep the peace, but rob, steal, kill, outrage women and children, and take away property and honor? The small lack of peace called war or the sword must set a limit to this universal, worldwide lack of peace which would destroy everyone.
This is why God honors the sword so highly that he says that he himself has instituted it [Rom. 13:1] and does not want men to say or think that they have invented it or instituted it. For the hand that wields this sword and kills with it is not man’s hand, but God’s; and it is not man, but God, who hangs, tortures, beheads, kills, and fights. All these are God’s works and judgments.
To sum it up, we must, in thinking about a soldier’s office, not concentrate on the killing, burning, striking, hitting, seizing, etc. This is what children with their limited and restricted vision see when they regard a doctor as a sawbones who amputates, but do not see that he does this only to save the whole body. So, too, we must look at the office of the soldier, or the sword, with the eyes of an adult and see why this office slays and acts so cruelly. Then it will prove itself to be an office which, in itself, is godly and as needful and useful to the world as eating and drinking or any other work.
There are some who abuse this office, and strike and kill people needlessly simply because they want to. But that is the fault of the persons, not of the office, for where is there an office or a work or anything else so good that self-willed, wicked people do not abuse it? They are like mad physicians who would needlessly amputate a healthy hand just because they wanted to. Indeed, they themselves are a part of that universal lack of peace which must be prevented by just wars and the sword and be forced into peace. It always happens and always has happened that those who begin war unnecessarily are beaten. Ultimately, they cannot escape God’s judgment and sword. In the end God’s justice finds them and strikes, as happened to the peasants in the revolt.5
As proof, I quote John the Baptist, who, except for Christ, was the greatest teacher and preacher of all. When soldiers came to him and asked what they should do, he did not condemn their office or advise them to stop doing their work; rather, according to Luke 3 [:14], he approved it by saying, “Rob no one by violence or by false accusation, and be content with your wages.” Thus he praised the military profession, but at the same time he forbade its abuse. Now the abuse does not affect the office. When Christ stood before Pilate he admitted that war was not wrong when he said, “If my kingship were of this world, then my servants would fight that I might not be handed over to the Jews” [John 18:36]. Here, too, belong all the stories of war in the Old Testament, the stories of Abraham, Moses, Joshua, the Judges, Samuel, David, and all the kings of Israel. If the waging of war and the military profession were in themselves wrong and displeasing to God, we should have to condemn Abraham, Moses, Joshua, David, and all the rest of the holy fathers, kings, and princes, who served God as soldiers and are highly praised in Scripture because of this service, as all of us who have read even a little in Holy Scripture know well, and there is no need to offer further proof of it here.
Martin Luther, AE 46.