This book analyzes the progress in inter-state security cooperation among Argentina, Brazil, and Chile (ABC) since 1983 as a consequence of these states' political democratization, economic liberalization, and sub-regional integration. The causal role of each of these variables has varied over time. Argentina's political democratization in 1983, followed by democratic transitions in Brazil in 1985 and Chile in 1990, ushered in security cooperation, ending a century-long phase of interstate rivalry and conflict management regimes. Economic liberalization adopted by the ABC countries from 1990 led, for the first time in the countries' history, to growing levels of economic, societal and political interdependence. This changed the countries' mutual threat perceptions and created incentives for largely bilateral conflict prevention regimes. This shift, together with the creation of Mercosur's customs union in 1995, opened a more advanced phase featuring sub-regional multilateral collective action in the security realm. Further advances will mostly depend on Mercosur's still unclear consolidation. Integration and security cooperation has been a deliberate state strategy during the 1990s, strengthening the ABC countries' capacity for domestic and international governance. The book concludes by asserting the need for more integrated theoretical frameworks able to articulate different levels of analysis and variations in causality.