This article explores whether and how internationalized prosecution contributes to the reduction of criminal violence. Internationalized prosecution is a strategy of cooperation between international organizations and domestic institutions to investigate and prosecute state security agents and criminals who collude to dominate illicit markets. We focus on the International Commission Against Impunity in Guatemala (CICIG), a UN-backed institution in which international investigators collaborated with Guatemala’s public prosecutors and the police to dismantle criminal structures that emerged during the country’s civil war. Results from a Synthetic Control Model show that had Guatemala not adopted the CICIG in 2008, the country’s homicide rate would have doubled in 2016. Substantively, the CICIG contributed to prevent over 18,000 murders. We argue that the training and protection the CICIG provided to Guatemalan prosecutors empowered them to reduce murder rates by preventing murder-for-hire operations, reducing criminal competition, deterring state-criminal collusion, restricting iron-fist policies, and discouraging private violence.