Meanwhile, Heraclius had been retaliating on Persia in the most effective way. In return for the invasion of Thrace by the Avars, he called in from beyond the Caucasus the wild Hunnish tribe of the Khazars, and turned them loose on Media and Assyria. Forty thousand of their horsemen laid waste the whole land, as far as the gates of Ctesiphon, and the emperor took possession of the upper valley of the Tigris, and prepared to strike at his rival’s capital in the coming year. [Sidenote: Battle of Nineveh, 627.] The campaign of 627 ended the triumphs of Heraclius. The last army of Persia, under a general named Rhazates, faced him near Nineveh. Charging at the head of the mailed horsemen of his guard, Heraclius slew the Persian chief with his own hand, and scattered his forces to the winds. The victorious army pressed on, and captured Dastagerd, the magnificent country-palace of Chosroes, near Ctesiphon, where they gained such plunder as no Roman army had won for many ages. They burnt Dastagerd, and four palaces more, while Chosroes fled eastward to conceal himself in the mountains of Susiana. The long-suffering Persians were at last growing tired of their arrogant lord. [Sidenote: Peace with Persia, 628.] His army rebelled against him, and proclaimed his son Siroes as king. Chosroes himself was thrown into a dungeon, where he perished of cold and starvation. The new king at once sent to ask for terms of peace from Heraclius. The emperor, knowing the exhaustion of his own realm, and its need for instant repose, made no hard conditions. Siroes restored all the Roman territory still in his hands, released all Roman captives, paid a war indemnity, and—greatest of all triumphs in the eyes of the subjects of Heraclius—gave back the ‘True Cross,’ and other spoils of Jerusalem. In May 628 the emperor was able to return to Constantinople, bringing peace and plenty with him. He had restored the boundary of the empire, and inflicted on Persia a blow from which she never recovered. His arms had penetrated far beyond the limits of the conquests of Trajan and Severus, and his six years of unbroken victory were a record which no Roman, save Julius Caesar, could rival. Not unjustly did the inhabitants of Constantinople receive him with chants and sacred processions, and hail him by the name of ‘the new Scipio.’ The crowning moment of his triumph came when the True Cross was uplifted in St. Sophia, and publicly exposed for the adoration of the faithful. Well might the emperor have sung his ‘_Nunc dimittis_’ on that day of solemn rejoicing, and prayed that the hour of his triumph might be the last of his life.