It is well known that the majority of militarized conflicts and wars have been fought by neighbors. Yet, much remains to be learned about the relationship between shared borders and militarized conflict. This article decomposes the effects of territorial contiguity into ex ante “observable” and “behavioral” effects. It provides powerful empirical evidence for the claim that although neighbors are more likely to experience conflict because of ex ante differences in observable variables such as economic interdependence, alliance membership, joint democracy, and the balance of military capabilities, most conflicts between neighbors occur because of differences in how neighbors and nonneighbors respond to the observable variables.