The current international system is based on Westphalian principles in which authority is defined territorially. Within this territory, the state has sole jurisdiction. Adherence to these principles has contributed to the decline of interstate war. Conversely, applying these principles and correlated norms to states that gained their independence after 1945 has contributed to civil conflicts. These norms are opaque, as is the case with the principle of self-determination; or they lock in an unstable status quo, as with uti possidetis , the principle that borders inherited at the moment of independence should always be maintained; or they are inconsistently applied and often violated, as with the principle of noninterference. Consequently, they provide poor guidelines as to when, and on which grounds, external intervention in civil wars might be warranted. I argue that the degree to which the combatants challenge Westphalian principles should guide policy responses. Furthermore, the international legal regime should reconsider uti possidetis . In some instances, partition might be a reasonable solution to civil wars.