This chapter introduces the National Security Institutions Data Set, an original cross-national resource offering the first systematic measurement of national security decision-making and coordination bodies across the globe from 1946 to 2015. The chapter leverages these data to probe the theory quantitatively, yielding three findings that are consistent with the theory’s propositions. First, it shows that national security institutions are more malleable than previous scholarship has suggested. Second, it finds that integrated institutions tend to perform better than institutional alternatives. Third, it shows that institutional change is associated with domestic environments in which leaders have political incentives to weaken the bureaucracy.