As soon as the ambitious mind of Artaxerxes had triumphed ever the resistance of his vassals, he began to threaten the neighboring states, who, during the long slumber of his predecessors, had insulted Persia with impunity. He obtained some easy victories over the wild Scythians and the effeminate Indians; but the Romans were an enemy, who, by their past injuries and present power, deserved the utmost efforts of his arms. A forty years' tranquillity, the fruit of valor and moderation, had succeeded the victories of Trajan. During the period that elapsed from the accession of Marcus to the reign of Alexander, the Roman and the Parthian empires were twice engaged in war; and although the whole strength of the Arsacides contended with a part only of the forces of Rome, the event was most commonly in favor of the latter. Macrinus, indeed, prompted by his precarious situation and pusillanimous temper, purchased a peace at the expense of near two millions of our money; but the generals of Marcus, the emperor Severus, and his son, erected many trophies in Armenia, Mesopotamia, and Assyria. Among their exploits, the imperfect relation of which would have unseasonably interrupted the more important series of domestic revolutions, we shall only mention the repeated calamities of the two great cities of Seleucia and Ctesiphon.

Seleucia, on the western bank of the Tigris, about forty-five miles to the north of ancient Babylon, was the capital of the Macedonian conquests in Upper Asia. Many ages after the fall of their empire, Seleucia retained the genuine characters of a Grecian colony, arts, military virtue, and the love of freedom. The independent republic was governed by a senate of three hundred nobles; the people consisted of six hundred thousand citizens; the walls were strong, and as long as concord prevailed among the several orders of the state, they viewed with contempt the power of the Parthian: but the madness of faction was sometimes provoked to implore the dangerous aid of the common enemy, who was posted almost at the gates of the colony. The Parthian monarchs, like the Mogul sovereigns of Hindostan, delighted in the pastoral life of their Scythian ancestors; and the Imperial camp was frequently pitched in the plain of Ctesiphon, on the eastern bank of the Tigris, at the distance of only three miles from Seleucia. The innumerable attendants on luxury and despotism resorted to the court, and the little village of Ctesiphon insensibly swelled into a great city. Under the reign of Marcus, the Roman generals penetrated as far as Ctesiphon and Seleucia. They were received as friends by the Greek colony; they attacked as enemies the seat of the Parthian kings; yet both cities experienced the same treatment. The sack and conflagration of Seleucia, with the massacre of three hundred thousand of the inhabitants, tarnished the glory of the Roman triumph. Seleucia, already exhausted by the neighborhood of a too powerful rival, sunk under the fatal blow; but Ctesiphon, in about thirty-three years, had sufficiently recovered its strength to maintain an obstinate siege against the emperor Severus. The city was, however, taken by assault; the king, who defended it in person, escaped with precipitation; a hundred thousand captives, and a rich booty, rewarded the fatigues of the Roman soldiers. Notwithstanding these misfortunes, Ctesiphon succeeded to Babylon and to Seleucia, as one of the great capitals of the East. In summer, the monarch of Persia enjoyed at Ecbatana the cool breezes of the mountains of Media; but the mildness of the climate engaged him to prefer Ctesiphon for his winter residence.

From these successful inroads the Romans derived no real or lasting benefit; nor did they attempt to preserve such distant conquests, separated from the provinces of the empire by a large tract of intermediate desert. The reduction of the kingdom of Osrhoene was an acquisition of less splendor indeed, but of a far more solid advantage. That little state occupied the northern and most fertile part of Mesopotamia, between the Euphrates and the Tigris. Edessa, its capital, was situated about twenty miles beyond the former of those rivers; and the inhabitants, since the time of Alexander, were a mixed race of Greeks, Arabs, Syrians, and Armenians. The feeble sovereigns of Osrhoene, placed on the dangerous verge of two contending empires, were attached from inclination to the Parthian cause; but the superior power of Rome exacted from them a reluctant homage, which is still attested by their medals. After the conclusion of the Parthian war under Marcus, it was judged prudent to secure some substantia, pledges of their doubtful fidelity. Forts were constructed in several parts of the country, and a Roman garrison was fixed in the strong town of Nisibis. During the troubles that followed the death of Commodus, the princes of Osrhoene attempted to shake off the yoke; but the stern policy of Severus confirmed their dependence, and the perfidy of Caracalla completed the easy conquest. Abgarus, the last king of Edessa, was sent in chains to Rome, his dominions reduced into a province, and his capital dignified with the rank of colony; and