A large literature has argued that domestic competition increases a militant organization's use and severity of terrorism to differentiate their “brand” and “outbid” other organizations. However, most empirical analyses infer such competition from the quantity of groups present in a geographic area. This approach neglects specific group relationships, such as cooperation, rhetorical or violent rivalry, or peaceful coexistence. We introduce a behavioral measure of group competition and argue that variation in the quality, rather than the quantity, of competition affects the violence profile of militant groups in unexpected ways. Violent competition, where militants attack one another, imposes significant constraints on group resources and increases groups’ dependence on civilian support, which exacerbates the costs of a popular backlash against brutality. Moreover, violent competition effectively substitutes for crowding out rivals via outbidding. As competition becomes extreme, we posit that groups increasingly opt for a strategy of terrorist restraint and reduce the share of high-profile attacks on soft civilian targets. We test this argument at the macro and micro levels with cross-national data on 290 organizations in civil war (1970–2018) and granular data on the subnational targeting strategy of the Islamic State in Syria (2013–2018). Both analyses provide robust support for our argument. The findings shed light on the strategic limitations of outbidding and provide important insights for research and policy.

Do rebel group violations of international humanitarian law during civil war—in particular, attacks on noncombatant civilians—affect conflict outcomes? I argue that in the post-Cold War era, rebel groups that do not target civilians have used the framework of international humanitarian law to appeal for diplomatic support from Western governments and intergovernmental organizations. However, rebel group appeals for international diplomatic support are most likely to be effective when the rebel group can contrast its own restraint toward civilians with the government's abuses. Rebel groups that do not target civilians in the face of government abuses, therefore, are likely to be able to translate increased international diplomatic support into more favorable conflict outcomes. Using original cross-national data on rebel group violence against civilians in all civil wars from 1989 to 2010, I show that rebel groups that exercise restraint toward civilians in the face of government violence are more likely to secure favorable conflict outcomes. I also probe the causal mechanism linking rebel group behavior to conflict outcomes, showing that when a rebel group exercises restraint toward civilians and the government commits atrocities, Western governments and intergovernmental organizations are more likely to take coercive diplomatic action against the government. The evidence shows that rebel groups can translate this increased diplomatic support into favorable political outcomes.