The Western defeat in Afghanistan was due to an inadequate process of strategic reflection informed, first, by an overestimation of the attractiveness of the Western political agenda to Afghans and, second, by overconfidence in the effectiveness of its military approach. As a corollary, popular support for the Taliban was underestimated. The insurgents possessed a degree of what we term strategic cohesion-a sociopolitical and military embeddedness within society-that produced a far stronger strategic effectiveness than we could replicate in our Afghan allies. Furthermore, a military-professional mindset underestimated the degree to which political considerations permeated the battlefield. The political effect of military actions was insufficiently integrated into strategic practice. Specifically, the linchpin officer in staff planning and field operations in Western armies struggled to act as what we term strategic colonels. In both respects, the war continues to offer important lessons for Western involvement in future conflict, including with Russia and China.