This paper examines changes in international security in the post cold war and their impact on three dimensions of South America's security: the institutional architecture, armed forces' missions, and ideas about security. The main argument is that changes in international security have encouraged a number of regional responses which reveal a remarkable level of activism and protagonism. International changes have become functional for the empowerment of traditional institutional practices, for the legitimation of old and new roles for the military and for the reinforcement of existing ideas on defense and security.