From Tertullian's De Corona (ca. 204 AD):
I think we must first inquire whether warfare is proper at all for Christians. ... Do we believe it lawful for a human oath [of military allegiance] to be superadded to one divine, for man to come under promise to another master after Christ, and to abjure father, mopther, and all nearest relatives, whom even the law has commanded us to honor and love next to God Himself? ... Should it be held lawful to make an occupation of the sword when the Lord proclaims that he who uses the sword shall perish by the sword? And shall the son of peace take part in battle when it does not even become him to sue at the law courts? And shall he apply the chain, the prison, and the torture, and the punishment who is not the avenger even of his own wrongs? ... Shall he carry a flag, too, hostile to Christ? ... You may see by a slight survey how many other offences are involved in the performance of military officers which we must hold to involve a transgression of God's law. The very carrying over of the name from the campt of light to the camp of darkness is a violation of it.
-- quoted in Robert W. Brimlow, What About Hitler? Wrestling with Jesus's [sic] Call to Nonviolence in an Evil World (Grand Rapids: Brazos, 2006), pp. 22-23.