The deployment of military personnel in response to urban crime has become increasingly salient in the United States. Yet the effectiveness and efficiency of such deployments remain largely unknown. This paper studies the effect of the August 2025 deployment of National Guard troops in Washington, DC, using administrative crime data and a triple differences design that compares changes before and after the August 11 deployment across treated and untreated areas, relative to analogous periods in prior years. The deployment reduced robberies, motor vehicle thefts, and thefts, had no detectable effect on homicides or burglaries, and increased assaults with dangerous weapons. A cost–benefit analysis implies 30-day benefits of $6.45 million against costs of approximately $90 million. For comparison, a community-based monitoring program in Chicago achieved similar crime-reduction benefits at less than four percent of the cost. This underscores the relative inefficiency of large-scale military deployments as a crime-control strategy.