The limits of the Roman empire still extended from the Western Ocean to the Tigris, and from Mount Atlas to the Rhine and the Danube. To the undiscerning eye of the vulgar, Philip appeared a monarch no less powerful than Hadrian or Augustus had formerly been. The form was still the same, but the animating health and vigor were fled. The industry of the people was discouraged and exhausted by a long series of oppression. The discipline of the legions, which alone, after the extinction of every other virtue, had propped the greatness of the state, was corrupted by the ambition, or relaxed by the weakness, of the emperors. The strength of the frontiers, which had always consisted in arms rather than in fortifications, was insensibly undermined; and the fairest provinces were left exposed to the rapaciousness or ambition of the barbarians, who soon discovered the decline of the Roman empire.
Chapter VIII: State Of Persia And Restoration Of The Monarchy.--Part I. Of The State Of Persia After The Restoration Of The Monarchy By Artaxerxes.
Whenever Tacitus indulges himself in those beautiful episodes, in which he relates some domestic transaction of the Germans or of the Parthians, his principal object is to relieve the attention of the reader from a uniform scene of vice and misery. From the reign of Augustus to the time of Alexander Severus, the enemies of Rome were in her bosom--the tyrants and the soldiers; and her prosperity had a very distant and feeble interest in the revolutions that might happen beyond the Rhine and the Euphrates. But when the military order had levelled, in wild anarchy, the power of the prince, the laws of the senate, and even the discipline of the camp, the barbarians of the North and of the East, who had long hovered on the frontier, boldly attacked the provinces of a declining monarchy. Their vexatious inroads were changed into formidable irruptions, and, after a long vicissitude of mutual calamities, many tribes of the victorious invaders established themselves in the provinces of the Roman Empire. To obtain a clearer knowledge of these great events, we shall endeavor to form a previous idea of the character, forces, and designs of those nations who avenged the cause of Hannibal and Mithridates.