A Fortune investigation reveals that the ATF never intentionally allowed guns to fall into the hands of Mexican drug cartels. How the world came to believe just the opposite is a tale of rivalry, murder, and political bloodlust. In the annals of impossible assignments, Dave Voth's ranked high. In 2009 the federal Bureau of Alcohol, Tobacco, Firearms and Explosives promoted Voth to lead Phoenix Group VII, one of seven new ATF groups along the Southwest border tasked with stopping guns from being trafficked into Mexico's vicious drug war. Some call it the "parade of ants"; others the "river of iron."...