75 A great deal depends on the causes for which men undertake wars, and on the authority they have for doing so; for the natural order which seeks the peace of mankind, ordains that the monarch should have the power of undertaking war if he thinks it advisable, and that the soldiers should perform their military duties in behalf of the peace and safety of the community. When war is undertaken in obedience to God, who would rebuke, or humble, or crush the pride of man, it must be allowed to be a righteous war; for even the wars which arise from human passion cannot harm the eternal well-being of God, nor even hurt His saints; for in the trial of their patience, and the chastening of their spirit, and in bearing fatherly correction, they are rather benefited than injured.
No one can have any power against them but what is given him from above. For there is no power but of God, who either orders or permits. Since, therefore, a righteous man, serving it may be under an ungodly king, may do the duty belonging to his position in the State in fighting by the order of his sovereign—for in some cases it is plainly the will of God that he should fight, and in others, where this is not so plain, it may be an unrighteous command on the part of the king, while the soldier is innocent, because his position makes obedience a duty,— how much more must the man be blameless who carries on war on the authority of God, of whom every one who serves Him knows that He can never require what is wrong?
Source: Reply to Faustus the Manichaean (New Advent)