Aman-Allah's effort to modernize Afghanistan in the first quarter of the twentieth century appears on the historic record as a failure. In this work we have sought to illuminate the nature of the problems that prevented success, especially those arising from the powerful institutions of the day. The error that was ultimately fatal to Aman-Allah's regime was his failure to appreciate the power of the 'ulama, pervasive in all levels of the culture, allowing them to become his single most effective opponent. This study has concentrated on the relationship between Aman-Allah and the 'ulama during the critical years 1919-1929. In the beginning Aman-Allah seemed to understand the force that could either support or be activated against him, and tried to work with the 'ulama, drawing their support through Islamic symbols such as jihad and pan-Islamism. The study shows how Aman-Allah initially won the support of the 'ulama by sponsoring the reintroduction of jihad against the British, in which the 'ulama played an important function, and by establishing himself as a major proponent of the pan-Islam movement. The 'ulama were pleased by his approach, with its marked contrast to the anti-clerical attitudes and practices of his grandfather Amir 'Abd al-Rahman. He was able to enlist some of the 'ulama to help formulate and support his reforms. Early in his regime, Aman-Allah had the majority support of the 'ulama. Despite his early popularity with the 'ulama, however, conflict emerged when Aman-Allah ventured to alter tradition regarding the family and society and to introduce radical modernization in Afghan society. Aman-Allah's attempt to recruit the support of the clergy and the tribal chieftains with whom they shared power was finally thwarted by his efforts at social reform. When government modernization deviated from tradition, a breach opened between state and clergy, manifested in the Khust Rebellion. The rift expanded in 1928 when Aman-Allah moved overtly toward secularization. The diverse elements among the 'ulama were solidified into a unitary force by this common threat. In the end, the 'ulama would no longer grant political legitimacy to Aman-Allah's regime, and with that recognition, it must fail.