Parmenio recommended that they should not attempt to pass the river immediately. The Persian army consisted chiefly of cavalry. Now cavalry, though very terrible as an enemy on the field of battle by day, are peculiarly exposed and defenseless in an encampment by night. The horses are scattered, feeding or at rest. The arms of the men are light, and they are not accustomed to fighting on foot; and on a sudden incursion of an enemy at midnight into their camp, their horses and their horsemanship are alike useless, and they fall an easy prey to resolute invaders. Parmenio thought, therefore, that the Persians would not dare to remain and encamp many days in the vicinity of Alexander's army, and that, accordingly, if they waited a little, the enemy would retreat, and Alexander could then cross the river without incurring the danger of a battle.