This chapter explains why Mexican drug cartels went to war as the country transitioned from authoritarian rule to electoral democracy. Postauthoritarian elites did not reform the military, the police, and the judicial system and did not dismantle the gray zone of criminality. Electoral mechanisms thus became catalysts of criminal violence. Subnational alternation and the rotation of parties in the gubernatorial seat undermined informal networks of protection that had allowed Mexican cartels to thrive, so they created their own private militias to defend themselves from rival groups and incoming opposition authorities, and to conquer rival territory. We use in-depth interviews with the first opposition governments and new data on historical patterns of government repression to show that state-level police and judicial authorities were key todeveloping informal networks of protection, allowing cartels to become major players in the international drug trafficking industry in the late 1980s. Using time-series, cross-sectional, and synthetic control models, we show that party alternation in the 1990s and early 2000s caused inter-cartel violence.