His death was a palpable loss to the Romans; but it by no means put an end to the ferment in the Parthian empire, and the Armenian war continued. Gabinius, after ending the Egyptian campaign, was just on the eve of turning to account the still favourable opportunity and resuming the interrupted Parthian war, when Crassus arrived in Syria and along with the command took up also the plans of his predecessor. Full of high-flown hopes he estimated the difficulties of the march as slight, and the power of resistance in the armies of the enemy as yet slighter; he not only spoke confidently of the subjugation of the Parthians, but was already in imagination the conqueror of the kingdoms of Bactria and India.
The new Alexander, however, was in no haste. Before he carried into effect these great plans, he found leisure for very tedious and very lucrative collateral transactions. The temples of Derceto at Hierapolis Bambyce and of Jehovah at Jerusalem and other rich shrines of the Syrian province, were by order of Crassus despoiled of their treasures; and contingents or, still better, sums of money instead were levied from all the subjects. The military operations of the first summer were limited to an extensive reconnaissance in Mesopotamia; the Euphrates was crossed, the Parthian satrap was defeated at Ichnae (on the Belik to the north of Rakkah), and the neighbouring towns, including the considerable one of Nicephorium (Rakkah), were occupied, after which the Romans having left garrisons behind in them returned to Syria. They had hitherto been in doubt whether it was more advisable to march to Parthia by the circuitous route of Armenia or by the direct route through the Mesopotamian desert. The first route, leading through mountainous regions under the control of trustworthy allies, commended itself by its greater safety; king Artavasdes came in person to the Roman headquarters to advocate this plan of the campaign. But that reconnaissance decided in favour of the march through Mesopotamia. The numerous and flourishing Greek and half-Greek towns in the regions along the Euphrates and Tigris, above all the great city of Seleucia, were altogether averse to the Parthian rule; all the Greek townships with which the Romans came into contact had now, like the citizens of Carrhae at an earlier time[4], practically shown how ready they were to shake off the intolerable foreign yoke and to receive the Romans as deliverers, almost as countrymen.
The Arab prince Abgarus, who commanded the desert of Edessa and Carrhae and thereby the usual route from the Euphrates to the Tigris, had arrived in the camp of the Romans to assure them in person of his devotedness. The Parthians had appeared to be wholly unprepared.
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.