Accordingly (701) the Euphrates was crossed (near Biradjik). To reach the Tigris from this point they had the choice of two routes; either the army might move downward along the Euphrates to the latitude of Seleucia where the Euphrates and Tigris are only a few miles distant from each other; or they might immediately after crossing take the shortest line to the Tigris right across the great Mesopotamian desert. The former route led directly to the Parthian capital Ctesiphon, which lay opposite Seleucia on the other bank of the Tigris; several weighty voices were raised in favour of this route in the Roman council of war; in particular the quaestor Gaius Cassius pointed to the difficulties of the march in the desert, and to the suspicious reports arriving from the Roman garrisons on the left bank of the Euphrates as to the Parthian warlike preparations. But in opposition to this the Arab prince Abgarus announced that the Parthians were employed in evacuating their western provinces. They had already packed up their treasures and put themselves in motion to flee to the Hyrcanians and Scythians; only through a forced march by the shortest route was it at all possible still to reach them; but by such a march the Romans would probably succeed in overtaking and cutting up at least the rear-guard of the great army under Sillaces and the vizier, and obtaining enormous spoil. These reports of the friendly Bedouins decided the direction of the march; the Roman army, consisting of seven legions, 4000 cavalry, and 4000 slingers and archers, turned off from the Euphrates and away into the inhospitable plains of northern Mesopotamia.

Far and wide not an enemy showed himself; only hunger and thirst, and the endless sandy desert, seemed to keep watch at the gates of the east. At length, after many days of toilsome marching, not far from the first river which the Roman army had to cross, the Balissus (Belik), the first horsemen of the enemy were descried. Abgarus with his Arabs was sent out to reconnoitre; the Parthian squadrons retired up to and over the river and vanished in the distance, pursued by Abgarus and his followers. With impatience the Romans waited for his return and for more exact information. The general hoped here at length to come upon the constantly retreating foe; his young and brave son Publius, who had fought with the greatest distinction in Gaul under Caesar[5], and had been sent by the latter at the head of a Celtic squadron of horse to take part in the Parthian war, was inflamed with a vehement desire for the fight. When no tidings came, they resolved to advance at a venture; the signal for starting was given, the Balissus was crossed, the army after a brief insufficient rest at noon was led on without delay at a rapid pace. Then suddenly the kettledrums of the Parthians sounded all around; on every side their silken gold-embroidered banners were seen waving, and their iron helmets and coats of mail glittering in the blaze of the hot noonday sun; and by the side of the vizier stood prince Abgarus with his Bedouins.

The Romans saw too late the net into which they had allowed themselves to be ensnared. With sure glance the vizier had thoroughly seen both the danger and the means of meeting it. Nothing could be accomplished against the Roman infantry of the line with Oriental infantry; so he had rid himself of it, and by sending a mass, which was useless in the main field of battle, under the personal leadership of king Orodes to Armenia, he had prevented king Artavasdes from allowing the promised 10,000 heavy cavalry to join the army of Crassus, who now painfully felt the want of them. On the other hand the vizier met the Roman tactics, unsurpassed of their kind, with a system entirely different.

His army consisted exclusively of cavalry; the line was formed of the heavy horsemen armed with long thrusting-lances, and protected, man and horse, by a coat of mail of metallic plates or a leathern doublet and by similar greaves; the mass of the troops consisted of mounted archers. As compared with these, the Romans were thoroughly inferior in the corresponding arms both as to number and excellence. Their infantry of the line, excellent as they were in close combat, whether at a short distance with the heavy javelin or in hand-to-hand combat with the sword, could not compel an army consisting merely of cavalry to come to an engagement with them; and they found, even when they did come to a hand-to-hand conflict, an equal if not superior adversary in the iron-clad hosts of lancers. As compared with an army like this Parthian one, the Roman army was at a disadvantage strategically, because the cavalry commanded the communications; and at a disadvantage tactically, because every weapon of close combat must succumb to that which is wielded from a distance, unless the struggle becomes an individual one, man against man. The concentrated position, on which the whole Roman method of war was based, increased the danger in presence of such an attack; the closer the ranks of the Roman column, the more irresistible certainly was its onset, but the less also could the missiles fail to hit their mark. Under ordinary circumstances, where towns have to be defended and difficulties of the ground have to be considered, such tactics operating merely with cavalry against infantry could never be completely carried out; but in the Mesopotamian desert, where the army, almost like a ship on the high seas, neither encountered an obstacle nor met with a basis for strategic dispositions during many days' march, this mode of warfare was irresistible for the very reason that circumstances allowed it to be developed there in all its purity and therefore in all its power. There everything combined to put the foreign infantry at a disadvantage against the native cavalry.

Where the heavy-laden Roman foot-soldier dragged himself toilsomely through the sand or the steppe, and perished from hunger or still more from thirst amid the pathless route marked only by water-springs that were far apart and difficult to find, the Parthian horseman, accustomed from childhood to sit on his fleet steed or camel, nay almost to spend his life in the saddle, easily traversed the desert whose hardships he had long learned how to lighten or in case of need to endure. There no rain fell to mitigate the intolerable heat, and to slacken the bowstrings and leathern thongs of the enemy's archers and slingers; there amidst the deep sand at many places ordinary ditches and ramparts could hardly be formed for the camp. Imagination can scarcely conceive a situation in which all the military advantages were more on the one side, and all the disadvantages more thoroughly on the other.

To the question, under what circumstances this new style of tactics, the first national system that on its own proper ground showed itself superior to the Roman, arose among the Parthians, we unfortunately can only reply by conjectures. The lancers and mounted archers were of great antiquity in the east, and already formed the flower of the armies of Cyrus and Darius; but hitherto these arms had been employed only as secondary, and essentially to cover the thoroughly useless Oriental infantry. The Parthian armies also by no means differed in this respect from the other Oriental ones; armies are mentioned, five-sixths of which consisted of infantry.

In the campaign of Crassus, on the other hand, the cavalry for the first time came forward independently, and this arm obtained quite a new application and quite a different value.