This topic-based module develops quantitative reasoning skills through a structured analysis of an important but accessible problem, imparting to students the appreciation that, for many questions/issues, a quantitative analysis can provide the insight and clarity that complements and moves beyond what might be gained through a qualitative approach. This module focuses on interstate war, and how its likelihood is affected by countries’ regime types. Does democracy cause peace between states? Known as the “democratic peace theory,” this hypothesis will be investigated by students using quantitative analyses. They will survey extant research and build empirical models to test the proposition.