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One of the most-debated issues about the orgy of drug-related violence in Mexico is whether the turmoil is confined to that country or is having a significant impact on the United States. The conventional wisdom is that although tensions have risen along the U.S.-Mexico border over the past five or six years, the actual impact of our southern neighbor’s carnage on communities in the United States has been minimal. Ioan Grillo, who has reported on developments in Mexico for more than a decade, concedes that Mexican cartels operate throughout the United States, but he insists “there has been no major spillover of violence from Mexico to its northern neighbor. As of 2011, after five years of cartel devastation south of the Rio Grande, the war simply hasn’t crossed the...