Some researchers suggest that the observed boom in the levels of violence in Mexico since 2008 are a consequence of placing federal military forces in states with a significant organized crime presence. However, little has been said about the role of the changeable, competitive, and violent nature of criminal organizations on this increasing violence. Using the literature on inter- and intra-state conflicts as matter of analogy to explain organized crime developments in Mexico, fragmentation and cooperation seem to be determinant forces that alter the equilibrium within Mexican criminal groups, affecting their territorial control. By using a private dataset gathered by the Drug Policy Program at the Center for Economic Research and Teaching (CIDE), we examine the evolution of criminal organizations in Mexico by focusing on their different alliances and fragmentations from December 2006 to December 2011. Our analysis suggests that violence is a consequence not only of the law enforcement actions, but also of the fragmentation and cooperation within and between private groups.