The withdrawal of Mali, Burkina Faso, and Niger from the Economic Community of West African States (ECOWAS) and their subsequent consolidation under the Alliance of Sahel States (AES) represents a critical juncture in West African regional security governance. This article examines the implications of this geopolitical rupture for counterterrorism cooperation and regional security architecture. Drawing on qualitative content analysis of primary documents, policy reports, and scholarly literature, the study reveals that the AES–ECOWASfragmentationhasfundamentally altered institutional frameworks for collective security, intelligence sharing, and joint military operations. While the AES emerged as a sovereignty-driven security response to perceived external pressures and insecurity, its formation has simultaneously weakened the cohesion of established regional institutions and created operational gaps exploitable by terrorist organizations. The analysis, grounded in Regional Security Complex Theory, demonstrates that complete security disengagement is neither practical nor sustainable given the transnational nature of contemporary security threats. The article concludes that the region's long-term security effectiveness depends on pragmatic cooperation mechanisms transcending political differences, while acknowledging the fundamental tension between national sovereignty and collective regional security.