CARRHAE, BATTLE OF, 53 B.C. Not since Cannae (q.v.) had the Romans suffered a defeat which made such a deep impres sion upon them as this, at the hands—or, more truly, the arrows —of the Parthians. And f our centuries were to pass before they sustained such another, at Adrianople (q.v.), this time irreparable in its more profound aspects. But as the damage wrought at Adrianople was more permanent than at Carrhae, so Carrhae was never retrieved as Cannae had been. For not only was the immediate effect of Carrhae to turn back the tide of Roman con quest, its ebb taking it as far back as the Aegean, but only one of their subsequent expeditions against Parthia achieved practical success, and that even was not long-lived. Crassus, triumvir with Caesar and Pompey, invaded Parthia with seven legions, totalling some 35,000 foot and 4,000 cavalry. On reaching the plains of Mesopotamia he was met by the Parthian general, Su rena, with an army composed entirely of horse-archers, mainly light cavalry but with a nucleus of heavy. To the foot-marching Roman legions they were an intangible foe, and not until he had penetrated several days' march beyond the Euphrates, through the desert, did Crassus make contact with them, and then appar ently with only a detachment of light cavalry, who served to screen the concentration of the main body. Crassus is said to have accepted the advice of his son for an immediate attack in contra diction to that of his other subordinates who urged that the army should encamp and rest, on the banks of the river Balissus. Upon the Roman advance the Parthians opened and, wheeling outwards, encircled the Roman army, and from a distance kept it under a continuous shower of arrows. Whenever the Romans sought relief by a charge of their own light troops, the Parthians fell back, only to close again and renew the arrow-storm as soon as these retired. Moreover, its moral and material effect was sus tained by the battle drill which, as with the Mongols later, was the basis of the Parthian tactics. This comprised a repeated "leap frogging" by which the forward troops were relieved by suc cessive echelons in rear, while those relieved replenished their quivers from camel-borne reserves. The younger Crassus ulti mately attempted a powerful charge with a mixed body of some 6,000 cavalry and foot to ease the pressure. But the Parthians lured it into a continued pursuit which drew it far from the main body, and then, holding it in front with a line of heavy cavalry, surrounded it with light horse, and kept it under fire until its ranks withered and died—all but a few hundred taken prisoners. The return of the victors to the main battlefield strained still more the morale of the sorely harassed Roman army, and al though still unbroken when nightfall brought temporary relief, its dissolution was only postponed. During the night it made a short retreat to the shelter of the walls of Carrhae, where it stayed during the day following. A further retreat was attempted the next night, but at daybreak it was assailed afresh and had to take refuge on a hill. Here Crassus was killed during negotiations for surrender and only an exhausted remnant of his army succeeded ultimately in recrossing the Euphrates. Carrhae saw the first great failure of the flexible infantry legion evolved from the Hanni balic war and from the reforms of Marius. The instrument of its downfall was the mobile horse-archer, and it is noteworthy that the eastern Roman empire came later to adopt this instrument in place of the legionary infantry as the staple of its military organ ization. This fact has all the more significance since the introduc tion in modern armies of the armoured fighting vehicle (see TANKS), for between this and the horse-archer, especially the armoured horse-archer, there is a close historical parallel just as there is between the campaign of Carrhae and the effects seen in the early exercises of experimental armoured forces. The par allel has struck the imagination of many of the advocates of the tank, but there is still greater significance in the sequel to Carrhae seen in the military organization of the Byzantines.