Do states experience more peace under female leadership? We examine this question in the context of Europe over the fifteenth to twentieth centuries. We use gender of the firstborn and presence of a sister among previous monarchs as instruments for queenly rule. We find that polities led by queens engaged in war more than polities led by kings. While single queens were more likely to be attacked than single kings, married queens were more likely to attack than married kings. These results suggest asymmetries in the division of labor: married queens were more inclined to enlist their spouses in helping them rule, which enabled them ultimately to pursue more aggressive war policies.