For the Primitive Muslim Arabs’ feat of conquering the Oriental provinces of the Roman Empire with one hand and the whole of the Sasanian Empire with the other hand simultaneously, between A.D. 632 and A.D. 643, see [reference given to an earlier part of the work]. In these twelve years of conquest the Arabs emulated the achievement of the Macedonians in 334-323 B.C. without quite equalling it. While the larger part of the area conquered was the same, the Arabs fell short of their Macedonian predecessors both on the north-west and on the north-east. On the north-west they did not win any permanent foothold in the Anatolian Peninsula; on the north-east they did not begin the conquest of the Oxus-Jaxartes Basin until more than half a century, or complete it until more than a century, had passed since their occupation of the north-eastern frontier fortresses of the Sasanian Empire in A.D. 643-51.
A Study of History, Vol VII, OUP, 1954 (footnote)