The current layered and bifurcated approach to homeland security and defense is insufficient for combating future threats to the nation. This book describes a new approach to homeland security and defense. Broadly speaking, this new approach, termed Domestic Security Cooperation, leverages the U.S. military's experience over the past ten years in Afghanistan and Iraq, and applies it to building capacity amongst organizations responsible for U.S. homeland security. Using the U.S. Army doctrinal Capabilities-Based Assessment model, the analysis highlights current capabilities, required capabilities, and solutions as they apply to organizations responsible for safeguarding the U.S.-Mexican Border. For the purposes of the assessment, these organizations include U.S. Northern Command, Customs and Border Protection, and Immigration and Customs Enforcement. The assessment, based on a scenario where extremist groups and Mexican-based transnational criminal organizations collude to infiltrate a weapon of mass destruction, highlights doctrinal, organizational, training, and policy changes necessary for a more effective approach in preventing illicit trafficking across the Southwest Border. Although the gap analysis and solutions focus on improvements between the interagency and Department of Defense as they apply to the Southwest Border, Domestic Security Cooperation is applicable to broader homeland security and defense initiatives.