Data on a large sample of interventions that individuals in two mountain gorilla (Gorilla gorilla beringei) groups made in agonistic interactions between others corroborate and extend earlier analyses in several ways. Related females supported each other as often as those in some female-bonded primates and maintained alliances while they resided together. Most unrelated females rarely supported each other, but some developed alliances. Females mostly supported other females with whom they had affinitive relationships against those they often engaged in dyadic aggression. They showed reciprocity in support, and often intervened against individuals who, in turn, often intervened against them. But even female coalitions that outnumbered their opponents by more than two-to-one had limited effectiveness, largely because males intervened in many female contests to control aggression. By rendering coalitions ineffective, males contribute to a combination of factors (e.g. low potential to gain from cooperation in contest feeding competition) that limit the benefits of female philopatry. Male curtailment of female aggression may influence female mate choice. Co-resident mature males in the study groups competed to control female aggression. High-ranking males curtailed aggression to females by subordinates, although two males formed an alliance against two others in their group. Immature animals mostly received defensive support against larger individuals and did not receive support from adults that could lead to a nepotistic dominance system.