This chapter traces the origin and development of terrorism in the Horn of Africa, linking it to the evolving security threats in West Africa and the Sahel. It argues that terrorism in the Sahel is a function of environmental, historical, and political factors, rooted in endogenous and exogenous variables. It notes that the region has proved a catchment area for various terrorist groups linked to Al-Qaeda and Islamic State following their significant incapacitation across the Levant. The consequence has been a web of complex security and humanitarian uncertainties across West Africa and the Sahel, characterised by sustained recurrent attacks on critical state institutions and infrastructure. Drawing from the regional context of terrorism in West Africa and the Sahel, the chapter notes that identity crisis and governance deficit have coalesced to shape a tragic course of history in ways that perpetuate state fragility and recurrence of violent conflict which are exploited by terrorist groups to expand their operations across the regions. It also examines factors that account for the resilience of terrorist groups in the regions, while highlighting the challenges of national and multilateral counterterrorism responses in West Africa and the Sahel.