Edward Campbell has kindly translated for us this letter of Isidore of Pelusium, from the Patrologia Graeca text. It came to my attention after a correspondent asked whether it referred to the Three Hundred Spartans.
To Esaias the soldier.
To the disorderly soldier.
If, from among your weapons, you consider your spears and your helmet and your breast-plate to be an assurance for your well-being, while you plunder and desolate the highways, know that many who had armed themselves more impregnably than you won for themselves a most lamentable death. Among us are recorded, on the one hand, Oreb, Zebah, Zalmunna, Abimelech and Goliath, and Absalom, and as many others who were like them. Among those outside, on the other hand, are the Hectors, the Ajaxes, and the Lacedaimonians themselves who, above all others, were prideful of their strength, since they did not possess justice in proportion to their power. If, then, you do not wish to be a worthless soldier, arrange yourself at once toward the spiritual war and wage war rather upon your own disorderliness.
 A more normal English rendering would give “Isaiah.”
 Possibly, “Against the disorderly soldier.”
 Or simply, “obtained.”
 i.e. Characters from the Old and New Testaments.
 See Judges 7, 8, and 9; 1 Samuel 17; 2 Samuel 13.
 i.e. Characters from Greek literature.
 i.e. the Spartans.
 Lit, “running along with their power,” a slightly confusing phrase. Isidore seems to mean that the Spartans’ power far surpassed their justice, hence they only had their power to be proud of.