ST. AUGUSTINE TO LEGATE BONIFACE

Do not think that no one can please God who is a soldier in military arms. Holy David was among these, to whom the Lord gave such great witness [cf. 1 Kings 14. 7, where God says David, "followed me with all his heart, doing only what was right in my eyes"] and many just men of that time among them. Among these was Cornelius [Acts 10] to whom the angel was sent.... Among these were those who came to John for baptism.... Surely he did not forbid them to serve in arms, to whom he ordered to be content with their pay.... . Some therefore fight for you by praying against invisible enemies; you work for them by fighting against visible barbarians.... So think first of this, when you arm yourself for battle, that even your bodily strength is a gift of God.... (c. AD 417 St. Augustine to Boniface, a Soldier Epistle 189, Paragraph 60)
source http://www.catholicmil.org/