We offer a theory explaining how alliances as international security regimes reduce military conflict between member-states through their internal provision of information concerning national military capabilities+ Bargaining models of war have shown that a lack of information about relative military capabilities functions as an important cause of war+ We argue that alliances provide such information to internal participants, and greater knowledge within the alliance about memberstate military capabilities reduces certain informational problems that could potentially lead to war+ This internal information effect, however, is a conditional one+ We posit that the information provided within the alliance matters most for dyads at or near power parity: the cases where states are most uncertain about who would prevail if a military conflict did emerge+ In power preponderant dyads where the outcome of a potential military conflict is relatively certain, the internal information provided by military alliances becomes less important+ Our statistical results provide strong support for these theoretical arguments+ In what is perhaps the largest research program in the discipline of international relations, scholars continue to explore the determinants of war and interstate military conflict+ Quantitative models in this research program commonly use a dyadyear unit of analysis and almost always include a variable for joint alliance membership+ Indeed, this variable represents a key part of what has become the "standard model" of interstate military conflict, 1 following Oneal and Russett's seminal paper on the subject+ 2 Our greatest thanks go to Ashley Leeds, who made available an advance copy of the ATOP 3+0 data set+ This article also benefited from presentations at the University of Wisconsin and at ISA-South in Columbia, S+C+ Finally, we thank Lisa Martin, two anonymous reviewers,

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