After wintering at Van, in the Armenian highlands, Heraclius dropped southward, in 625, and came into regions more within the ken of Western historians. He recovered the long-lost fortresses of Amida and Martyropolis, the ancient bulwarks of the empire on the upper Tigris, which had been for nearly twenty years in Persian hands, and once more picked up his communication with Constantinople, which had almost lost sight of him during the two last campaigns. The year ended with a fourth crushing defeat of Shahrbarz, who had endeavoured to throw himself between the emperor and his homeward path by defending the passage of the Sarus, near Germanicia. But 626 was destined to be the decisive year of the war. Before acknowledging himself beaten, the obstinate Chosroes was determined to make one final effort. Drawing every man that he could together, for the Persian empire was now growing exhausted, the old king made two armies of them. While the larger was left in Mesopotamia and Armenia, to endeavour to keep Heraclius employed, a great body under Shahrbarz slipped southward, round the emperor’s flank, and marched for the Bosphorus. [Sidenote: Great Siege of Constantinople, 626.] Chosroes had concerted measures with the treacherous Chagan of the Avars for a combined attack on Constantinople, from both the European and the Asiatic side of the strait. When Shahrbarz appeared at Chalcedon, he found the Avars already masters of Thrace, and preparing to beleaguer Byzantium. The two armies could see each other across the water, but they were wholly unable to communicate with each other; for the Roman fleet kept such excellent guard in the straits that no boat could cross. The patrician Bonus made a most gallant defence, the garrison was adequate, and the population kept a good heart, for they knew that the Persian was striking his last desperate blow. Heraclius himself was so well satisfied with the impregnability of his capital that he only sent a few veteran troops by sea to co-operate in the defence, and kept the greater part of his army in hand for an attack on the heart of the dominions of Chosroes. Meanwhile, the host of Shahrbarz had to look on in helpless impotence, while the Avars, on the other side of the Bosphorus, made their attempt on Constantinople. On the night of the 3d of August 626 the Chagan gave the signal for the assault. A body of Slavs, in small boats, attempted to storm the sea-wall from the side of the Golden Horn, while the main body of the Avars moved against the land-wall. But the galleys of Bonus rammed and sunk the light vessels of the Slavs, and the assault of the Avars miscarried entirely. Thereupon the Chagan hastily broke up his camp, and retired beyond the Balkans. The siege was practically raised, though the army of Shahrbarz still remained encamped at Chalcedon. Thus ended the first of the four great sieges of Constantinople of which we have to tell.