From grassroots terrorism to the nuclear ambitions of hostile nations, the United States faces increasingly complex threats to its national security, and combating these threats continues to demand a reshaping of the nation's security structure, military forces and defence expenditures. In this study, the author offers a detailed analysis of critical challenges affecting U.S. national security and how failures in adapting to these challenges have exacerbated the strains on available resources. He systematically identifies the most glaring obstacles to successful national security planning and proposes constructive and practical ways to proceed in the future. The book focuses on specific challenges, each addressed within the context of the Iraq War, the Afghan War, the 'War on Terrorism', and the risk of conflicts with North Korea and China. Out of the lessons drawn from these experiences, he examines the future of international coalitions, asymmetric warfare, nation building, and stability operations, and concludes that perhaps the most pressing area for change is the need for accountability among civilian and military policymakers.