From Afghanistan: A Cultural and Political History, by Thomas Barfield (Princeton U. Press, 2010), Kindle pp. 10-11:
Chapter 2 examines the premodern patterns of political authority and the groups that wielded it. During this period nation-states did not exist and regions found themselves as parts of various empires. This chapter focuses on how (and what kinds of) territory was conquered, how conquerors legitimated their rule, and the relationship of such states with peoples at their margins.
In Turko-Persia, rulers did not seek to impose their authority uniformly across the landscape. Instead they imposed direct rule only in urban areas and on productive agricultural lands that paid more than it cost to administer them. They employed strategies of indirect rule when dealing with the peoples who had poor subsistence economies. These did not repay the cost of administration, and their location in remote mountains, deserts, and steppes provided natural bulwarks against attack. But the relationship between the center and these hinterlands was of great significance because when state authority weakened, it was tribal groups from the hinterlands that most often toppled existing regimes. The tribal groups that most commonly succeeded at this task were the Turks of central Asian steppe origin. Their hierarchical tribal structure gave them an advantage over more egalitarian tribal groups, which had more difficulty unifying and supporting a single leader. The Turks were also heirs to a horse cavalry tradition that remained militarily decisive against people who fought on foot until gunpowder weapons entered the picture.
The long-term dominance of Turkish dynasties in the region has been underplayed in a modern Afghan history that gives primacy to the Pashtuns as the country’s rulers. But in reality the Pashtuns were never rulers in Afghanistan before the mid-eighteenth century. Only at that time, after serving as military auxiliaries to the Safavid and Afsharid empires in Iran, did the Durrani Pashtuns come to power by adopting the governmental structure and military organization of their former overlords. Indeed Ahmad Shah Durrani, the founder of the Afghan Empire, inherited the lands he ruled only after his Iranian patron, Nadir Shah Afshar, was assassinated. He and his heirs imposed the Turkish tradition of royal succession that demanded the ruler be chosen from only within the royal lineage. During this period the Afghan Empire slowly lost its most valuable provinces and retreated into the boundaries similar to those of today’s Afghanistan.