This book focuses on the following questions. Why does a relatively low level of trust characterize U.S.-Mexican defense relations? Has the long-shared history of the two policies helped or hindered the building of trust? What are the main obstacles to the strengthening a military-to-military partnership based on trust? In particular, what should the U.S. military do to ensure better cooperation between both militaries to meet the security challenges confronting North America and beyond in the twenty-first century? This book will determine why prior U.S.-military engagements with the Mexican military have been ineffective in shaping a relationship based on trust. This research study highlights the historical and cultural paradigms that have challenged the relationship between the U.S. and Mexican militaries. The focus of this research is not to blame the professionalism and effectiveness of the Mexican military to combat these security challenges, but to study a policy environment, and provide policy recommendations of trust-building mechanisms to be incorporated (from the U.S. side) to help build a solid relationship built on trust, not capabilities. Finally, this study addresses key factors that have prevented a trust-building program, and will outline a range of policy options that the U.S. military forces can use to build a much-needed trust between these two institutions.