This monograph is part of a broader study of U.S. security roles in sub-Saharan Africa that is meant to inform ongoing e?orts to restructure the U.S. military command arrangements in Africa and to posture air and space capabilities, as well as other U.S. government assets, to more e?ectively protect and advance American national interests on the continent. The monograph deals with the rising threat to U.S. interests represented by the spread of militant Islamism and the development of radical Islamist networks in East Africa. Equally important, however, it seeks to place those trends in the broad context of the diverse currents of Islamic practice in the region and the social, economic, and political factors that have shaped the security environment in East Africa.The monograph examines the complex ethno-religious landscape in East Africa, the characteristics of the East African environment that have produced failed or weak states susceptible to exploitation by extremist groups, and the factors that have contributed to the emergence of these groups. It concludes with a series of recommendations for countering or mitigating the e?ects of adverse trends and protecting U.S. interests in East Africa.