Res Gestae

However, in order not to be wholly inactive, nor be criticised for slackness, he ordered Arbitio and Agilo, commanders of the cavalry and of the infantry, to sally forth promptly with strong forces, not with a view of provoking the Persians to battle, but to draw a cordon on our bank of the Tigris and be on the watch to see where the impetuous king might break through. Moreover, he often warned

them by word of mouth and in writing that if the enemy’s horde should begin to cross, they were to retreat quickly.