Res Gestae

When we marched on from this place, the Persians, since their frequent losses made them dread regular battles with the infantry, laid ambuscades, and secretly attended us, from the high hills on both sides watching our companies as they marched, so that the soldiers, suspicious of this, all day long neither raised a palisade nor fortified themselves with stakes.

And while the flanks were strongly protected and the army, as the nature of the ground made necessary, advanced in square formation, but with the battalions in open order, it was reported to the emperor, who even then unarmed had gone forward to reconnoitre, that the rear guard[*](Arma cogentium = agmen cogentium, with arma = armatos, as often in Ammianus. Cf. xvi. 2, 10, where arma cogentes probably has the same meaning as arma cogentium in this passage.) had suddenly been attacked from behind.