This article considers the advances of a research project about Comparative Defense Systems, sponsored by the Social Sciences and Humanities University Center of the University of Guadalajara, Mexico. The first section discusses different understandings of the concept of security, because of its ambiguity and complexity, and the missions of the Armed Forces. The second section takes on the role of the Mexican military, based on its missions and functions. In particular, special treatment in given to the direct participation of the military in police and security forces functions (a tendency called militarization of public security); efforts to combat narcotics trafficking and the counter-insurgency war, which allows the Mexican Armed Forces acquire arms training and combat experience, which is considered an important qualitative factor. Finally, the third section questions the future of national defense and its military instrument in Mexico through analysis of the lack of correspondence between national defense and foreign policy; the operational organization, struture and deployment of the Armed Forces; the human factor; the lack of a joint doctrine; the defense system and internal security, among other topics. The paper ends with some unanswered questions in the subject, in hopes of generating public debate about the role of the Armed Forces and a clear definition of National Security.