On a sudden, the Osroenes whom Abgar had not carried with him attack the Romans in the rear,[733] and at the same time appear, glittering in the sun, the long lines of the cuirassed horsemen. Crassus then forms his army in a square. Each face is composed of twelve cohorts, and the rest is in reserve. The cavalry and light infantry, divided into two corps, flank two opposite sides of the square.[734] Publius and Cassius command, one the right, the other the left. Crassus takes his place in the centre.[735] The heavy cavalry, lance in rest, charge the great Roman square, and attempt to break it; but the thick and close ranks of the legions oppose an invincible resistance. The Parthians fall back a certain distance and call up their numerous archers, then, all together, they return in line, and throw upon the deep masses of the Romans a shower of missiles of which none fail of their aim. The legionaries, if they remain in their position, have the disadvantage of their _pila_ and slings, which carry but a short distance, and, if they advance to use their swords, they lose that cohesion which forms their strength. Without moving, and defending themselves with difficulty, they see their numbers diminish without being discouraged; they hope that the enemy will soon have exhausted his munitions. But the ranks of the Parthians succeed each other; as quickly as the first have used all their arrows, they go to fetch others near a long line of camels which carry their provisions. The combat has lasted several hours; and the Parthians continue to extend their circle, and threaten to surround entirely the great Roman square. In this critical position, Crassus can only have recourse to his cavalry. The side hardest pressed by the enemy is that commanded by Publius; his father orders him to make a desperate effort to disengage the army.