Some object that this command of Christ is altogether inconsistent with civil life in Commonwealths; Who, say they, would suffer, when he could hinder it, the pillage of his estate by an enemy; or would not repay the evil suffered by a plundered province of Rome on the plunderers according to the rights of war? But these precepts of patience are to be observed in readiness of the heart, and that mercy, not to return evil for evil, must be always fulfilled by the will.
Yet must we often use a merciful sharpness in dealing with the headstrong. And in this way, if the earthly commonwealth will keep the Christian commandments, even war will not be waged without good charities, to the establishing among the vanquished peaceful harmony of godliness and righteousness. For that victory is beneficial to him from whom it snatches license to sin; since nothing is more unfortunate for sinners, than the good fortune of their sins, which nourishes an impunity that brings punishment after it, and an evil will is strengthened, as it were some internal enemy.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Epist., 138, 2