The Cambridge History of India, Vol. I: Ancient India v1.c25.35

v1.c25.35

There was nothing to arrest this army between the point they had now reached and the Indus. The local chieftains had indicated their submission. All along the north side of the Kābul however lay the hills, whose inhabitants in their rock citadels, in the valleys of the Kūnar, the Panjkora, and the Swāt, were unschooled to recognise an overlord, and as prepared to give trouble to anyone who tried to incorporate them in an imperial system as their Pathān successors of a later day. But it was not Alexander's way to leave unsubdued regions beside his road. His army therefore broke up into two divisions. One, commanded by Hephaestion, the king's friend, and Perdiccas, the proudest of the Macedonian nobles, moved to the Indus by the most direct route. This would probably mean a route along the south bank of the Kābul, whether through the actual Khyber Pass or not¹; the other, led by the king himself, turned up into the hills. The two divisions were to rejoin each other upon the Indus; Hephaestion and Perdiccas, arriving there first, it was calculated, would have made all preparations for the passage of the great river.