Res Gestae

And for one who was traversing unknown regions greater precautions against hidden dangers were necessary, since the craft and many wiles of the nation were to be feared. Therefore the emperor, with light-armed skirmishers, now took his place at the head of the army, and now brought up the rear; and in order that no hidden danger might escape his notice, he scanned the rough thickets and valleys, using either his native affability or threats to keep his men from scattering too loosely or too far.

However, he allowed the enemy’s fields, abounding in fruits of every kind, to be set on fire with their crops and huts, but only after each man had fully supplied himself with everything that he needed; and in this way the safety of their foes was impaired before they knew it.