Arrian wrote: “Alexander now took the soldiers who had been left behind in his pursuit and advanced into Hyrcania, which is the country lying on the left of the road leading to Bactra. On one side it is bounded by lofty mountains densely covered with wood, and on the other it is a plain stretching as far as the Great Sea in this part of the world. He led his army by this route, because he ascertained that the mercenaries serving under Darius had succeeded in escaping by it into the mountains of Tapuria; at the same time he resolved to subdue the Tapurians themselves. [Source: Arrian the Nicomedian (A.D. 92-175), “Anabasis of Alexander”, translated, by E. J. Chinnock, London: Hodder and Stoughton, 1884, gutenberg.org]
“Having divided his army into three parts, he himself led the way by the shortest and most difficult route, at the head of the most numerous and at the same time the lightest division of his forces. He despatched Craterus at the head of his own brigade and that of Amyntas, some of the archers, and a few of the cavalry against the Tapurians; and he ordered Erigyius to take the the rest of the cavalry, and lead the way by the public thoroughfare, though it was longer, conducting the wagons, the baggage, and the crowd of camp-followers. After crossing the first mountains, and encamping there, he took the shield-bearing guards together with the lightest men in the Macedonian phalanx and some of the archers, and went along a road difficult and hard to travel upon, leaving guards for the roads wherever he thought there was any peril, so that the barbarians who held the mountains might not at those points fall upon the men who were following.
Having passed through the defiles with his archers, he encamped in the plain near a small river; and while he was here, Nabarzanes, the commander of Darius's cavalry, Phrataphernes, the viceroy of Hyrcania and Parthia, and the other most distinguished of the Persians in attendance on Darius, arrived and surrendered themselves. After waiting four days in the camp, he took up those who had been left behind on the march, all of them advancing in safety except the Agrianians, who, while guarding the rear, were attacked by the barbarian mountaineers. But these soon drew off when they got the worst of it in the skirmish. Starting from this place, he advanced into Hyrcania as far as Zadracarta, the capital of the Hyrcanians. In this place he was rejoined by Craterus, who had not succeeded in falling in with the Greek mercenaries of Darius; but he had thoroughly traversed the whole country, gaining over part of it by force and the other part by the voluntary capitulation of the inhabitants. Erigyius also arrived here with the baggage and wagons; and soon after Artabazus came to Alexander with three of his sons, Cophen, Ariobarzanes, and Arsames, accompanied by Autophradates, viceroy of Tapuria, and envoys from the Greek mercenaries in the service of Darius.
“To Autophradates he restored his viceregal office; but Artabazus and his sons he kept near himself in a position of honour, both on account of their fidelity to Darius and because they were among the first nobles of Persia. To the envoys from the Greeks, begging him to make a truce with them on behalf of the whole mercenary force, he replied that he would not make any agreement with them; because they were acting with great guilt in serving as soldiers on the side of the barbarians against Greece, in contravention of the resolution of the Greeks. He commanded them to come in a body and surrender, leaving it to him to treat them as he pleased, or to preserve themselves as best they could. The envoys said that they yielded both themselves and their comrades to Alexander, and urged him to send some one with them to act as their leader, so that they might be conducted to him with safety. They said they were , in number. Accordingly he sent Andronicus, son of Agerrhus, and Artabazus to them."
Edited from : factsanddetails.com