Crossing a Mexican drug cartel usually comes with a price — death. On the U.S.- Mexico border, that price is being paid daily with an endless stream of execution-style slayings in a war to control drug routes to the north. As Mexican officials crack down on these cartels, violence spreads. Lives are merely the cost of doing business, and since 2007 the international press has documented more than 7,400 drug war-related murders on the border. Police are gunned down in public squares. Failed drug smugglers are tortured to death and bound from head to toe in duct tape. Enemies are...