History demonstrates that actions of violent non-state actors are internationally consequential. Yet the question about the conditions under which violent non-state actors (VNSAs) can present a real challenge to great powers has been under-theorized. Pursuing a dual goal of theory building and theory testing, this paper conceptualizes “effectiveness” of violent non-state actors and discusses the extent to which great powers are vulnerable to violent non-state actors under different circumstances. Hypotheses are tested qualitatively and conclusions are based on examples drawn from two country case studies of Russia and Great Britain. Specifically, cases include medieval piracy, anti-imperial movements and mutinies, Fenians, the IRA, and revolutionary and anarchist movements. Findings show that VNSAs’ influence is more complex and extensive than suggested by mainstream IR literature. Specifically, depending on the extent of “slack” resources available to great powers, VNSAs can not only exacerbate great power decline but also can retard and arrest great power rise. ..PAT.-Conference Proceeding [ABSTRACT FROM AUTHOR]