 Since the nascent Caliphate regarded itself as the heir of the Sassanid (and to a lesser extent, Byzantine) tradition and "civilizational consciousness", the Arab caliphs also adopted the notion that, in the words of the historian Gerald Mako, it was their duty "to protect the settled, i.e. the civilized world from the northern barbarian". To this imperative was added the Muslim concept of division of the world into the "House of Islam" (Dar al-Islam) and the "House of War" (Dar al-Harb), to which the pagan Turkic nomads were consigned. The two armies consequently represented two different military philosophies. While the Umayyad armies disposed of a sizeable cavalry contingent, both light and heavy, their mainstay was the infantry, to the extent that in battle the Arab cavalry was often limited to skirmishing during the initial phases, before dismounting and fighting on foot. The Khazars on the other hand, although they had adopted elements of the civilizations of the south and possessed towns, remained a largely tribal and semi-nomadic power. Like their Central Asian counterparts, they practiced a highly mobile form of warfare, relying on their highly skilled and hardy cavalry. As the historian Khalid Yahya Blankinship writes, "the Khazars were to prove difficult opponents for the Muslims, perhaps partly because their state was not highly organized and thus did not have a center whose fall would bring about a sudden collapse and rapid surrender". The eastern Caucasus range became the main theatre of the Arab – Khazar conflict, with the Arab armies aiming to gain control of Derbent (known in Arabic as Bab al-Abwab, "Gate of Gates") and the Khazar cities of Balanjar and Samandar, whose location has yet to be established with certainty by modern researchers. Both of the latter are referred to as Khazar capitals by different Arab writers, and may have functioned as winter and summer capitals respectively. It was only later, under the impact of the Arab attacks, that the Khazars moved their capital further north, to Atil on the mouths of the Volga.
