Protective interventions by a third party on the behalf of others are generally admired, and as such are associated with our notions of morality, justice and heroism1-4. Indeed, stories involving such third-party interventions have pervaded popular culture throughout recorded human history, in myths, books and movies. The current developmental picture is that we begin to engage in this type of intervention by preschool age. For instance, 3-year-old children intervene in harmful interactions to protect victims from bullies5, and furthermore, not only punish wrongdoers but also give priority to helping the victim6. It remains unknown, however, when we begin to affirm such interventions performed by others. Here we reveal these developmental origins in 6- A nd 10-month old infants (N = 132). After watching aggressive interactions involving a third-party agent who either interfered or did not, 6-month-old infants preferred the former. Subsequent experiments confirmed the psychological processes underlying such choices: 6-month-olds regarded the interfering agent to be protecting the victim from the aggressor, but only older infants affirmed such an intervention after considering the intentions of the interfering agent. These findings shed light upon the developmental trajectory of perceiving, understanding and performing protective third-party interventions, suggesting that our admiration for and emphasis upon such acts-so prevalent in thousands of stories across human cultures-is rooted within the preverbal infant's mind.