Why do some militant groups wage sustained armed conflicts (or insurgencies) while other groups do not? I argue that shifts in nascent rival relations between militant groups, from competition to consolidation, are key to understanding this puzzle. A militant group which has consolidated its rivals - whether by destruction, merger, or hegemonic dominance - should be in a stronger position to fight the target state than groups preoccupied with counterproductive violence against rivals within a militant movement. This thesis uses a multi-method, three-stage, research design starting with a novel quantitative regression analysis of 246 prominent militant groups worldwide from 1970- 2007, featured in the Global Terrorism Database (GTD). I find that, on average, organizational characteristics (i.e. ideology and organizational structure) and constituency dominance are stronger indicators for engagement in sustained insurgencies than traditional proxies for observable group capabilities (i.e. group size, state sponsorship, etc.), challenging conventional wisdom. The second stage narrows in on a more bounded population (Middle Eastern and North African insurgent groups) and uses cross-case comparative methods to build my theory based on three forms of primary rival relations: competition (infighting or outbidding), strategic alliance, and hegemonic consolidation. I then use process tracing methods to explore within-case inferences and identify causal mechanisms in three diverse case studies: Hezbollah, the Kurdistan Workers' Party (PKK), and the Syrian Muslim Brotherhood. Rival consolidation helps dominant groups mobilize resources effectively and overcome two major organizational hurdles: collective action and principal-agent problems. Without major competitors, dominant groups attract recruits and support, while militant leaders divert their attention to strengthening organizational capacity and preparing for war with the target state. Hegemonic militant groups are also in a stronger position to ensure compliance among rank-and-file operatives, attract outside support, and secure critical safe haven to sustain an insurgency. My temporal theory of rival consolidation offers a more compelling explanation that accounts for the timing of sustained insurgency onset, compared to scholarly accounts that rely on largely static factors or remain incomplete. From a policy perspective, this thesis challenges assumptions and presents a generalizable framework identifying nascent rival relations as a pragmatic indicator that can help practitioners anticipate potential insurgent threats.