One of the primary functions of modern states is to prevent domestic unrest and threats to their rule. Political scientists have argued that limitations on state power and authority are important pre-conditions for civil conflict. However, scholars have overlooked the territorial dimensions of state strength. The power of the government to repress challengers is largely limited to its own geographic territory while opposition groups frequently mobilize across national boundaries. Opportunities to mobilize dissident activities abroad, particularly in neighboring states, make rebellions more likely to emerge and endure. Extraterritorial mobilization also exacerbates informational problems, making it more likely that negotiations to resolve conflict will fail. Neighboring states are more likely to host rebel organizations if they have a pre-existing rivalry with the target government or if they are too weak to prevent rebel access. Moreover, large refugee diasporas often contribute to conflicts at home. Transnational rebellions will also internationalize civil conflicts by creating tensions between states and increasing the likelihood of a state-to -state dispute. Finally, conflict resolution is best served by regional cooperation to combat insurgents and/or facilitate negotiations. Several hypotheses are developed relating to the onset and duration of civil war as well as the propensity for interstate conflict. These are tested using quantitative analyses as well as more detailed examinations of the civil wars in Nicaragua and Rwanda