Numbers 31:3

And Moses spake unto the people, saying, Arm some of yourselves unto the war, and let them go against the Midianites, and avenge the LORD of Midian.

Ver. 3. Arm some of yourselves unto the war.] Lactantius a being, according to his name, a mild and gentle man, abhorred bloodshed, thought it not lawful for a just man to be a warrior, whose justice was his warfare. But this was his error; Patres legendi sunt cum venia; God bids here, Arm yourselves, &c. Indeed it is utterly unlawful for men wilfully to thrust themselves into unnecessary wars: and it is reported in the life of Augustine, that he would never pray for such. But when God sounds the alarm, as here, "Cursed is he that doeth this work of the Lord negligently: Cursed is he that keepeth back his sword from blood." Jer 48:10

a Instit., lib. vi. cap. 20.