A comparison, functionally and structurally, of the United States' two most important alliances--NATO and the Organization of American States (OAS)--based on the premise that alliances are calculated ventures in which each partner expects certain returns for his investment. The purpose is to learn more about alliances in general and to consider possible changes in one organization in view of experiences in the other. The analysis indicates that more precise theories of supranationality and integration are needed in the study of alliances. OAS is a supranational organization and NATO is an integrated organization. Either could proceed toward more integration or toward less. In the case of NATO, an easing of integration could be in U.S. interests over the next 10 years. In the case of the OAS, its supranational character has enabled it to operate as an effective collective security agency. Integration, if properly introduced, could supplement the present system. It could never substitute for supranationality as it has in NATO.