Although social difficulties have been identified as sequelae of children’s experiences with interparental conflict and insecurity, little is known about the specific mechanisms underlying their vulnerability to social problems. Guided by emotional security theory, this study tested the hypothesis that children’s emotional insecurity mediates associations between interparental conflict and their social difficulties by undermining their affiliative goals in best friendships. Participants included 235 families with the first of 5 measurement occasions over a 10-year period occurring when children were in kindergarten (mean age = 6 years). Findings from the lagged latent difference score analyses indicated that intensification of multi-method assessment of interparental conflict during the early school years predicted subsequent increases in children’s emotional insecurity 5 years later in adolescence. In the latter part of the cascade, rises in emotional insecurity predicted decreases in adolescent friendship affiliation, which, in turn, were specifically associated with declines in social competence. The specificity of this cascade of changing processes in predicting social problems was supported by the robustness of the findings after the inclusion of static measures of each construct as predictors, parent-child relationship insecurity as a covariate, and increases in children’s internalizing symptoms as an alternative outcome.