Roy also describes regional, ethnic and sectarian variations in mujahideen organization. In the Pashtun areas of the east, south and southwest, tribal structure, with its many rival sub-divisions, provided the basis for military organization and leadership. Mobilization could be readily linked to traditional fighting allegiances of the tribal "lashkar" (fighting force). In favorable circumstances such formations could quickly reach more than 10,000, as happened when large Soviet assaults were launched in the eastern provinces, or when the mujahideen besieged towns, such as Khost in Paktia province in July 1983. But in campaigns of the latter type the traditional explosions of manpower—customarily common immediately after the completion of harvest—proved obsolete when confronted by well dug-in defenders with modern weapons. Lashkar durability was notoriously short; few sieges succeeded.