By the beginning of the eleventh century the crescent of Islam had flown from the Iberian heartlands to northern India. How can this rapid and remarkable expansion be explained? Most historians point to a combination of the Arabs’ military advantages and the political weaknesses of their opponents. The Byzantine and Sassanid Empires had just fought a grueling century-long war and had also been weakened by the plague. Equally important are the military strength and tactics of the Arabs. For example, rather than scattering as landlords of peasant farmers over conquered lands, Arab soldiers remained together in garrison cities, where their Arab ethnicity, tribal organization, religion, and military success set them apart. All soldiers were registered in the diwān (dih-WAHN), an administrative organ adopted from the Persians or Byzantines. Soldiers received a monthly ration of food for themselves and their families and an annual cash stipend. In return, they had to be available for military service.