Understanding the factors responsible for successful interactions between cultures has been an ongoing investigation among anthropologists, social workers, and organizational psychologists. The need for employees who are able to function effectively across cultures has resulted in a great deal of research examining which factors enable expatriate effectiveness. Despite the necessity of a workforce that is able to function across cultures in today’s global economy, an even greater case can be made for cross-cultural competence (3C) in the U.S. military. The potential for loss of life and international-level consequences is high if our military forces are not adequately prepared. This is why the Department of Defense has identified 3C as a critical determinant of success for military missions. Despite the critical need for military 3C, a review of the literature found no validated instruments developed to assess the readiness of our troops to work closely with foreign nationals and coalition forces in the context of military deployments. As such, the overarching goal of this validation study was to enable the U.S. military to prepare and train its forces in 3C, specifically allowing the military to: (1) better assess troop readiness to engage other cultures; (2) target training to those skills that help achieve missions in the field; (3) design more authentic cross-cultural training exercises; (4) assess the effectiveness of crosscultural training; and (5) guide the development of future cultural training efforts. To that end, a blended approach to scale development was undertaken, whereby critical-incident interviews with subject matter experts informed which of the individual difference predictors from the civilian literatures would likely be applicable to the military domain. Initial administration of the prototype instrument to 792 military members, followed by exploratory factor analysis, revealed six hypothesized factors of 3C. Following scale development, the Cross-Cultural Competence iv Inventory (3CI) was administered to almost 5,000 service members, and the six-factor structure was confirmed as well as cross-validated. Another data collection effort focused on assessing the stability of the six factors over time, via test-retest reliability analysis. A final validation study revealed Cultural Exploration to be a significant predictor of three of the four performance criteria, as rated by supervisors on deployment. Furthermore, this study offered the unique perspective gained by administering two popular civilian instruments along with a military-based tool, providing insight into the nature of military 3C and the ways in which it is similar to, and distinct from, civilian 3C. Additionally, important theoretical contributions may help guide future empirical research and military applications. This study is the initial step in assessing readiness for cultural interaction in the military. The results may serve to guide future efforts in military research in order to support our forces in the field as well as to guide the military establishment in making decisions on training, education, and operations in the context of mission success.