Whether neutral or on the side of a combatant, third-party states’ intervention in ongoing interstate conflicts is a triadic phenomenon which involves ties between a joining state and the two originators of the dispute. Existing studies on this topic have failed to fully capture the triadic nature of intervention, preferring instead to focus either on the joiner’s motivations or on the distinct dyadic relationships between joiners and the two separate combatants. Building on classic structural theories of triadic balance and on prior work by Maoz et al. (2007), in this article we address the triadic aspect of both mediation and “joining behavior”. The nature of the triadic relations among disputants and third parties influences not just the likelihood of intervention, but also the type of intervention. When triadic relations are unbalanced, third parties are more likely to intervene as intermediaries. On the contrary, when triadic relations are balanced, third parties are more likely to intervene in a partisan manner. We explore our main hypotheses by constructing a triadic data set that combines Corbetta and Dixon’s (2005) data on partisan third-party interventions and Frazier and Dixon’s (2006) data on neutral (intermediary) interventions in militarized interstate disputes with a friendship–hostility scale extracted from international events data (IDEA and COPDAB).