The U.S. military cooperates more with the Colombian military than the Mexican military in combating drug trafficking in the Western Hemisphere. This thesis analyzes the international relations theories of liberalism, realism, and constructivism to help explain why. Historical relationships matter in cooperation. Mexican and U.S. military units waged war to defend and take territory from one another. Mexico passed a constitution banning a garrison of foreign military units within Mexico, leading to low cooperation. The Colombian and U.S. militaries defended the Panama Canal during World War II to keep the shipping lanes open, and Colombia allows a garrison of U.S. military personnel in Colombia, leading to greater cooperation. Realism best explains reasons for when and why these two countries cooperate with the United States. Cooperation exists when there are shared external security concerns by the two countries. Cooperation exists when the internal instability of one country creates a reliance on another country. Cooperation remains low when there is no common external security threat, when one state perceives the other as a threat, or when a country can control internal stability on its own. Further cooperation with Mexico will depend on U.S. military leaders' willingness to empathize with Mexicans about past U.S. military interventions. Further cooperation with Colombia will require continued military-to-military relationships to form, followed by agreements to solidify those relationships. I. INTRODUCTION * A. LITERATURE REVIEW * 1. Cooperation and Realism * 2. Cooperation and Liberalism * 3. Cooperation and Constructivism * B. POTENTIAL EXPLANATIONS AND HYPOTHESES * 1. Realist Hypothesis * 2. Liberal Hypothesis * 3. Constructivist Hypothesis * C. RESEARCH DESIGN * D. METHODOLOGY * II. EVALUATING COOPERATION BETWEEN THE UNITED STATES AND MEXICAN MILITARIES * A. INTRODUCTION * B. PRIOR TO WORLD WAR II * C. WORLD WAR II TO THE WAR ON DRUGS * D. WAR ON DRUGS TO 9/11 * E. 9/11 TO THE PRESENT * F. ANALYSIS * G. CONCLUSION * III. EVALUATING COOPERATION BETWEEN THE UNITED STATES AND COLOMBIAN MILITARIES * A. INTRODUCTION * B. PRIOR TO WORLD WAR II * C. WORLD WAR II TO THE WAR ON DRUGS * D. WAR ON DRUGS TO 9/11 * E. 9/11 TO PRESENT * F. ANALYSIS * G. CONCLUSION * IV. CONCLUSION AND ANALYSIS * A. ANALYZING THE TWO CASE STUDIES * 1. U.S. and Mexican Militaries * 2. U.S. and Colombian Militaries * B. CONTINUED U.S. COOPERATION WITH COLOMBIA * C. ADJUSTING U.S. COOPERATION WITH MEXICO * D. FURTHER RESEARCH