AP Photo

On January 30, 2010, a commando of at least 20 hit men parked themselves outside a birthday party of high school and college students in Villas de Salvarcar, Ciudad Juarez. Near midnight, the assassins, later identified as hired guns for the Mexican cartel La Linea, broke into a one-story house and opened fire on a gathering of nearly 60 teenagers. Outside, lookouts gunned down a screaming neighbor and several students who had managed to escape. Fourteen young men and women were killed, and 12 more were wounded before the hit men finally fled. Indirectly, the United States government played a role in the massacre by supplying some of the firearms used by the cartel murderers. Three of the high caliber weapons fired that night in Villas de Salvarcar were linked to a gun tracing...