In the ongoing debate concerning whether democracies can carry out effective national security policy, the role of transparency costs has received little attention. I argue for a more nuanced understanding of how some democracies that possess specific investigative institutions, such as national security–relevant freedom of information laws, legislative oversight powers, and press freedoms, are able to avoid the problems of which democracy skeptics warn. Using a new dataset on national security accountability institutions in democracies within a Bradley-Terry framework, I find that national security oversight mechanisms raise the probability that a democracy wins international disputes as well as increasing the expected number of enemy casualties, as compared to democracies that lack effective oversight. Contra previous theories of foreign policy efficacy, I find that the chances for democratic foreign policy success are maximized when competitive elections are linked to institutions that increase the retrospective revelation of previously classified information.