It is well documented that democratic nations very rarely engage in militarized conflict with other free nations. This study expands on this notable pattern by examining the possibility that democracies may also be more amenable to collaborate in militarized interstate conflict. A theoretical rationale for this expectation is developed resting on the cooperative norms of democratic political culture. With empirical analysis encompassing all nations from the Napoleonic era to the end of the Cold War, it is found that joint democracy has a positive and robust impact on the probability that two states will be on the same side at the start of a militarized interstate dispute. This conclusion remains robust after accounting for the potent influences of major power status, geographic proximity, dyadic alliance, and regime maturity on the probability of militarized collaboration. The results suggest that researchers may wish to seek answers for democratic foreign behavior that encompass the realm of interstate cooperation as well as the often-noted absence of interdemocratic militarized conflict.