Why do some rebel groups adopt diverse portfolios of violence, while others appear to specialize in a single form of contention at a time? In this article, I theorize that rebel groups expand violent repertoires in response to changes in pressure levied upon them by the state and rival challengers. Just as competition fosters innovation in commercial firms, vying over a monopoly on violence promotes diversity in rebel group behavior. By varying targets and the use of direct and indirect force, rebels can alleviate external pressure, increase operational tempo and lethality. However, there is a threshold of utility for this diversification, beyond which groups will reduce the number of simultaneous strategies. The amount of violence needed to reach this threshold is greater for states that use indiscriminate rather than selective violence. I find consistent support for these propositions after integrating event data from the Global Terrorism Database (GTD) and the Armed Conflict Location & Event Data Project (ACLED) to generate measures of state pressure and challenger diversity for 779 rebel groups in 42 civil wars in 31 countries between 1997 and 2020. These findings have important implications for research and policy that aims to explain rebel group behavior and evaluate the efficacy of countermeasures.