The rise of the Central Asian empire of the Turks proper (Tu-Kueh) dates from their overthrow of the Juan-Juan in Mongolia in 552, under their great Khan, Mokan. His brother Istämi (the Silzibul of the Byzantine historians), the semi-independent jabghu of the ten tribes of Western Turks, after consolidating his power in the Ili and Chu valleys, formed an alliance with Khusrū Anūshīrwān, and in a joint attack between 563 and 568 the two powers completely overthrew the Ephthalite kingdom and divided their territories. For a brief moment the Oxus was the actual boundary between Īrān and Tūrān; under pressure from the silk traders of Sogdiana, however, the alliance was broken and the weaker successors of Anūshīrwān could scarcely do more than maintain their outpost garrisons on the Murghāb. From this time the Ephthalites, like the Kushans, were gradually assimilating to the Iranian population[5], though the change was less rapid in the Cisoxine lands of Lower _T_ukhāristān, Bādghīs, and Herāt, where Ephthalite principalities were re-constituted, probably with Turkish support, and continued to give Persia much trouble on her north-eastern frontiers[6]. On the other hand the Turks of the five western tribes (Nu-she-pi), who became independent after the break up of the Great Khanate about 582, maintained their suzerainty over Sogdiana and the middle Oxus basin by frequent expeditions, in one case at least as far as Balkh. There is no trace in our records of extensive Turkish immigration into the conquered lands; at most, small groups of Turks wandered south with their herds, especially, it would seem, south of the Iron Gate[7]. In general, Turkish interference in the administration of the subject principalities was at first limited to the appointment of military governors and the collection of tribute. Thus, in the semi-legendary account given by An-Naysābūrī of the Turkish conquest of Bukhārā the Bukhār Khudāh is represented as the chief dihqān under the Turkish governor. It is possible also that the native princes maintained guards of Turkish mercenaries.