Violent conflict has enduring effects on child health, but the speed at which these effects manifest is not fully understood. By use of data from a decade before to shortly after the Nigerian Boko Haram insurgency extended across the border to Cameroon, this study investigates the immediate effects of a deteriorating security environment caused by foreign-born insurgent terrorism on children’s health. Boko Haram attacks decrease weight-for-height for children under 5 years—an indicator of short-term health and nutrition—within an average of 2.6 months after the attacks. This effect is likely driven by a reduction in health-care service utilization, which can exacerbate the prevalence and the severity of conditions such as fever and diarrhea. However, the attacks do not affect dietary diversity or child mortality. The results underscore the importance of maintaining accessibility to health-care service after the start of terrorist violence to prevent irreversible effects, a concern that is increasingly relevant for countries combating the infiltration of foreign terrorists.