Chapter 2 introduces two concepts that form the core of the theoretical argument presented in the book: settled borders and commitment problems. The concept of border settlement can encompass either mutual agreement on or mutual satisfaction with (i.e., acceptance of) a territorial border division, and scholars sometimes slip between these two meanings. Our argument relies on a conceptualization of border settlement as the mutual agreement of a border’s delimitation under international law. We also argue that the lack of border settlement contributes to interstate rivalry via a commitment problem. We offer an innovation within the rivalry program by connecting rivalries to a theory of bargaining breakdown. In particular, we argue that many unsettled borders are the product of a commitment problem. Commitment problems are a negotiating obstacle often resolved through war. Yet we propose that states might manage commitment problems through interstate rivalries as well. The second part of the chapter therefore explains the origins of commitment problems and why initiating and maintaining a rivalry might be a valid method for managing certain subsets of them.