This chapter explains why the federal government’s decision to deploy the military in a War on Drugs against the cartels led to a five-fold increase in violence. We argue that acute political polarization and partisan conflict between Right and Left, and the bitterly contested 2006 presidential election, motivated the War on Drugs.The federal government developed interventions in states where the president’s conservative co-partisans ruled, but deliberately neglected leftist governors – the president’s main political rivals – and then blamed the violence on them. We use time-series cross-sectional models to show that between 2007 and 2012 drug violence was more intense in leftist-governed municipalities, not because of incompetence but due to conflict between a Right-wing federal government and leftist governors. We present a natural experiment comparing the intensity of criminal violence in nearly identical municipalities located across the state borders of Michoacán (ruled by the Left) and Jalisco (ruled by the Right). This partisan conflict explains why municipalities on the Michoacán side of the border experienced significantly higher levels of violence than those in Jalisco.