GUATEMALA CITY — Guatemalan drug boss Juan Jose "Juancho" Leon was summoned by Mexican traffickers for what he was told was business. Instead, dozens of attackers ambushed his entourage with grenades and assault rifles, killing Leon and 10 others in a brazen demonstration of power. Mexican drug traffickers are branching out as never before — spreading their tentacles into 47 nations, including the U.S., Guatemala and even Colombia, long the heart of the drug trade in Latin America. The expansion comes amid a military crackdown in Mexico and the arrests of major Colombian suppliers and poses a new challenge for...