At this period, therefore, so far from the Oxus being a barrier, there was considerable intercommunication between the peoples on either side, and at least the elements of a racial and cultural unity, in spite of political divisions. This is a factor of importance in the history of the Arab conquests: the conquest of Transoxania is intimately linked with the fortunes of Lower _T_ukhāristān, and only became possible when the latter country was completely subdued. On the other hand, the Jaxartes formed a natural racial and political frontier. “Shāsh and _S_ughd have seldom run together” says Vámbéry, and in spite of nominal annexations on more than one occasion Muslim rule was not effectively imposed on Shāsh and Farghāna until some time after the final conquest of Transoxania. Their chief importance for the history of Transoxania is that they formed the jumping-off place for counter-invasions from the East. It is not without significance that of the two battles which were decisive in establishing Arab rule in Sogdiana one was fought to the west of Balkh and the other on the Talas river, far into the Turkish lands beyond the Jaxartes (see pp. 84 and 96). _Political Divisions._ Researches into Chinese records have now made it possible to obtain a more definite idea of the political conditions of these frontier provinces in the seventh century. All the principalities acknowledged the Khan of the Western Turks as overlord and paid tribute to him under compulsion, though, as will appear, there is good cause for doubting whether a Turkish army ever came in response to their appeals for support until the rise of the Türgesh power in 716.