This CMI Working Paper focuses on the relationship between the military, civilian governments and civilian populations in Mexico. It highlights key dynamics since the Mexican revolution (1910-1917) and up until the Ayotzinapa incident in 2014, revealing that the Mexican military throughout the 20th Century has co-existed with civilian governments in a climate of mutual distrust yet co-dependence. The report also shows that the Mexican military has consistently been involved in repression of the civilian populations, from the clamp-down on “subversive groups” in the context of the Cold War to the civilian suffering produced by the so-called “War on Drugs” of today. These trajectories have in the present produced a crisis of legitimacy both for the security forces as well as for the Mexican state vis-a-vis its populace.