Chapter 5 presents a theory of borders and rivalry onset. When a border region contains attributes capable of affecting state power, a commitment problem is more likely to develop. The commitment problem undermines the settlement process by threatening the stability of any settlement agreement that might be signed, and therefore, encourages the involved states to eschew reaching an agreement in the first place, leaving borders unsettled and disputed. States then invest in foreign policy tools to compete over the insecure border, in the hopes of gaining a future bargaining advantage that will allow them to overcome the commitment problem and achieve a favorable and durable settlement. The competition produces a rivalry relationship between the states. We derive three hypotheses connecting unsettled borders, power endowments, and rivalry relations. In order to address alternative theoretical explanations, four additional hypotheses are presented relating rivalry initiation to regime type, power relations, alliances, and ethnic identify claims.