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 member-state
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.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, Scott
Gehlbach, Chuck Gochman, Zaryab Iqbal, George Krause, Pat McDonald, Jon
Pevehouse, Bill Reed, Kevin Sweeney, and Harrison Wagner for their
detailed comments and/or suggestions.