What explains the puzzling dynamics of inter-group militant relations? In this study, I develop a single theoretical framework to simultaneously explain militant cooperation and infighting by specifying the conditions under which competition over constituency and territory leads to cooperation instead of infighting. I argue that shared constituency and territorial presence facilitate cooperation by promoting collective identity, lengthening the shadow of the future, justifying the alliance in the eyes of the constituency, and lowering the costs of joint action. However, when groups with shared constituencies and territorial presence draw on the same social networks to recruit militants and strive to exert hegemony over the same territorial stronghold, competition over resource mobilization starts threatening the groups’ chances of survival. I test my expectations using social network analysis tools on a novel time-series network database of cooperative and adversarial relations between 52 ethnonationalist Northeast Indian militant groups from 1990 through 2021. I find that (1) groups with no shared constituency or territorial presence are unlikely to engage with each other, (2) groups with shared constituencies and territorial presence are likely to cooperate as long as they can differentiate their recruitment pools and territorial strongholds, whereas (3) groups that draw on the same recruitment pool and those whose strongholds overlap are likely to fight. The results also underscore that interactions among a set of groups affect interactions among other sets of groups.