Abstract How can defense cooperation with process-tracing methodology be explained? To answer this question, this paper tests the ability of process-tracing explanations in the case of the Brazilian proposal for the creation of the Union of South American Nation’s (UNASUR) South American Defense Council (CDS). Combining process-tracing methodology with a case study research design, this paper analyzes which causal pathways, mechanisms, timing of events, and policies best explain Brazilian behavior.Keywords: Defense Cooperation, South American Defense Council, Brazilian Foreign Policy, UNASUR. Received: May 26, 2017 Accepted: August 18, 2017 Introduction T he study of defense and international security plays a significant role in the establishment of International Relations as an academic field. Issues of war and peace, but also of cooperation, have occupied a central stage in this effort. In this area of inquiry, Latin and South America has provided an extensive proving ground for theories and explanations of defense and security cooperation (Buzan and Waever 2003; Hurrell 1998; Kacowicz 1998; Mares 1997; 2001). Along with the numerous institutions and regimes created to foster cooperation, some countries have occasionally played a decisive role in the maintenance or changing of patterns of defense cooperation in the region. In this paper, Brazil and its proposal for the creation of the South American Defense Council (CDS) is the case study.Brazil participates in a broad range of institutions, organizations, and regimes that promote defense and security cooperation at both the hemispheric and regional level. This net