This article hits hard in illuminating the ways in which the US has contributed to and caused the violence in Mexico. Structural adjustment policies, arms sales, and demand for cocaine are a few of the ways. Here is a highlight:
Dube, Dube, and García-Ponce found that the lifting of the U.S. federal ban led to a “substantial increases in homicides … tied specifically to guns” in the non-California Mexican border zones, with murders rising “60% more in municipios at the non-California entry ports” in Texas, New Mexico, and Arizona.  Mexico suffered “at least 238 additional deaths annually in the area located within 100 miles of the border ports” in the non-California border zones in Mexico. Today the Mexican cartels routinely load up on U.S. military-grade weapons with larger clips. The easy availability of U.S. weapons made for war has transformed much of Northern Mexico a war zone.