The great king then ransomed the neighbouring cities of Chalcis and Apamea, and recrossed the Euphrates into Mesopotamia. Here, where strong and well-armed fortresses blocked his way, Chosroes found that he could effect nothing; after looking at Edessa he found it too strong, and made his way to Daras. To this town he laid siege, but was beaten off without much difficulty, and then returned home for the winter (540). The Persians were never destined to win again such successes as had fallen to them in this the first year of the war. By the next spring Justinian had reinforced the eastern frontier with all his disposable troops, and the mighty Belisarius himself had arrived to take command of the army of Mesopotamia. But it was not fated that the great king and the great captain should ever measure themselves against each other. Hearing that the frontier to the south was now well guarded, the Persian had resolved to make a dash at a new point of the Roman line of defence. While expected on the Euphrates he quietly marched north through the Median and Iberian mountains, crossed many obscure passes, and appeared on the Black Sea coast by the river Phasis. The Romans here held the shore by their great castle of Petra, while the Lazi, the tribes of inland Colchis, were Roman vassals. Chosroes overran the land, constrained the Lazi to do him homage, and, after a short siege, took Petra. Meanwhile Belisarius, on finding the Persian invasion of Mesopotamia delayed, had crossed the frontier in the far south, beaten a small Persian force in the field, and ravaged Assyria from end to end, though he could not take the great fortress of Nisibis. On hearing of this raid Chosroes returned from Colchis with his main army, whereupon Belisarius retired behind the ramparts of Daras. The campaign had not been eventful, but the balance of gain lay on the side of the Persians, whose frontier now touched the Black Sea. Nor was the next year (542) destined to see any decisive fighting. Belisarius had concentrated his army at Europus on the Euphrates and waited to be attacked, but save one raid no attack came, though Chosroes had brought the full force of his empire up to Nisibis. The Roman chronicles ascribe his sluggishness to a fear of the reputation of Belisarius, but another cause seems to have been more operative. The great plague of 542 had just broken out in Persia, and its ravages were probably the real cause of the retreat and disbanding of the army of Chosroes, much as in 1348 the ‘black death’ caused English and French to drop for a time their mutual hostilities.