St. Athanasius on War

St. Athanasius the Great ca. 297-373

[I]t is not right to kill, yet in war it is lawful and praiseworthy to destroy the enemy; accordingly not only are they who have distinguished themselves in the field held worthy of great honors , but monuments are put up proclaiming their achievements. So that the same act is at one time and  under some circumstances unlawful, while under others, and at the right time, it is lawful and permissible. (Letter 48)