The German tribes had somehow or other been kept quiet since the time of Marcus; but with the Parthians there was twice war. Once the emperor led his army against Adiabene, the country east of the Tigris, and Arabia, which, like Osroëne, Media, and others, were distinct vassal kingdoms under Persian supremacy: this campaign, Severus conducted without being at war with the Parthians themselves. The second time, however, he directly attacked the Parthians; and then was the flourishing city of Ctesiphon, which the Parthians had built over against Seleucia to humble it, taken and sacked by Severus: it is strange that he did not make this country a province. He made peace, and gave back Babylon; but kept Adiabene, and more especially Mesopotamia, subject to his supremacy: under Marcus the Euphrates had been the boundary river. The Roman emperors had always to wage war, owing to the very immensity of the empire which otherwise would have sunk into utter effeminacy. He had afterwards another war besides in Britain, and it is surprising that he should have thought it necessary to bring such vast forces of imperial Rome against the weak Caledonian barbarians on the Scottish border. In this war, he took with him his two sons, the elder of whom, Caracalla, was at that time twenty-two years old, while Geta was several years younger: the former was with him as his colleague, the other as Cæsar (he is the first who is mentioned on inscriptions with the title of _nobilissimus_). Before his death, he also raised both of them to be _Augusti_, and made them heirs of the empire. Severus had by his own power caused himself to be adopted as the son of M. Aurelius, without meaning thereby to deceive any one, except perhaps the lowest of the people; it being merely a fiction by which he wanted to designate himself as the lawful possessor of the empire, calling himself _M. Antonini filius, T. Pii nepos_, and so on as high up as Nerva: he therefore gave his eldest son, M. Bassianus, the name of M. Antoninus. This name, or _Divus Antoninus, Imperator noster Antoninus, Antoninus Magnus_, is in the Pandects always to be understood of this Caracalla. That last appellation is in fact so generally bestowed on him only by the moderns: in the _Scriptores Historiæ Augustæ_ it is met with only once, and that in the form Caracallus, which is a popular nickname: I am very loth to use it. Both of the young princes were the sons of Julia Domna, a Syrian woman whom Severus is said to have married because she was recommended to him by the astrologers, as her horoscope pointed out that she was destined to be a princess.[54] Julia was a remarkable person: she was a woman of great cleverness, but of very lax morals. She has, however, atoned for her faults by her misfortunes.