This multimethod study of 101 mothers, fathers, and children elucidates poorly understood role of children's attachment security as "moderating" a common maladaptive trajectory: from parental power assertion, to child resentful opposition, to child antisocial conduct. Children's security was assessed at 15 months, parents' power assertion observed at 25 and 38 months, children's resentful opposition to parents observed at 52 months, and antisocial conduct rated by parents at 67 months. Moderated mediation analyses indicated that in insecure dyads, parental power assertion predicted children's resentful opposition, which then predicted antisocial conduct. This mechanism was absent in secure dyads. Early insecurity acts as a catalyst for a dyad embarking on mutually adversarial path toward antisocial outcomes, whereas early security defuses this maladaptive trajectory.