I regret that several works which are indispensable for a thorough study of the subject have, for linguistic reasons, been inaccessible to me. Such are van Vloten’s _Opkomst der Abbasiden_, and almost the whole range of Russian research work. Through the kindness of Sir Denison Ross, however, I have been able to avail myself of a draft MS. translation of the most important and valuable of them all, Professor W. Barthold’s _Turkestan_, as well as of his as yet unpublished London lectures on “The Nomads of Central Asia.” My sincere thanks are due to Sir Denison Ross also for his continued interest and material assistance ever since he first introduced me to the subject; to Sir Thomas Arnold for much encouragement and helpful counsel; to Professor Barthold, who has read the MS. through and made a number of valuable suggestions; to the Trustees of the Forlong Bequest Fund for their kindness in undertaking the publication; and in no small measure to my wife, who has given much time and labour to preparing the MS. for publication. London, April, 1923. I. INTRODUCTION THE OXUS BASIN _Early History._ The Oxus is a boundary of tradition rather than of history. Lying midway between the old frontier of Aryan civilisation formed by the Jaxartes and the Pamīr and the natural strategic frontier offered by the north-eastern escarpment of the plateau of Īrān, it has never proved a barrier to imperial armies from either side. It was not on the Oxus but on the Jaxartes that Alexander’s strategic insight fixed the position of Alexander Eschate, and when the outposts of Persian dominion were thrust back by the constant pressure of the Central Asian hordes, their retreat was stayed not on the Oxus but on the Murghāb. Thus when the tide of conquest turned and the Arabs won back her ancient heritage for Persia, they, like Alexander, were compelled to carry their arms ever further to the East and all unknowing re-establish the frontiers of the Achaemenid Empire. It was from the legends of Sāsānian times, enshrined in the pages of the historians and the national epic of Firdawsī, that the Oxus came to be regarded as the boundary between Īrān and Tūrān. Through all the centuries of invasion, however, the peoples of Sogdiana and the Oxus basin remained Iranian at bottom, preserving an Iranian speech and Iranian institutions. But the political conditions of the country at the period of the Arab conquests were so complex that it is necessary to trace briefly the course of their development.