Where cross-border sanctuaries enable rebels to marshal external support, classical theories of counterinsurgency extol the strategic value of border fortification. By sealing borders, counterinsurgents can erode transnational militants’ resources, degrading the quality of rebellion. Extending resource-centric theories of conflict, I posit a fortification dilemma inherent in this strategy. Externally-supplied rebels can afford conventional attacks and civilian victimization. When border fortifications interdict their foreign logistics, insurgents compensate by cultivating greater local support. In turn, rebels prefer more irregular attacks and cooperative relations with civilians. Hence, counterinsurgent border fortification trades-off reduced rebel capabilities for greater competition over local hearts-and-minds. I test this theory using declassified microdata on border fortification and violence in Iraq. Results highlight the central link between border control and cross-border militancy, and show how governments can contest the transnational dimensions of civil wars, like external rebel sponsorship.