One afternoon last August, at a hospital on the outskirts of Los Angeles, a former beauty queen named Emma Coronel gave birth to a pair of heiresses. The twins, who were delivered at 3:50 and 3:51, respectively, stand to inherit some share of a fortune that Forbes estimates is worth a billion dollars. Coronel’s husband, who was not present for the birth, is a legendary tycoon who overcame a penurious rural childhood to establish a wildly successful multinational business.

If Coronel elected to leave the entry for “Father” on the birth certificates blank, it was not because of any dispute over patrimony. More likely, she was just skittish about the fact that her husband, Joaquín Guzmán, is the C.E.O. of Mexico’s Sinaloa cartel, a man the Treasury Department recently described as the world’s most powerful drug trafficker. Guzmán’s organization is responsible for as much as half of the illegal narcotics imported into the United States from Mexico each year; he may well be the most-wanted criminal in this post-Bin Laden world. But his bride is a U.S. citizen with no charges against her. So authorities could only watch as she bundled up her daughters and slipped back across the border to introduce them to their dad.

Known as El Chapo for his short, stocky frame, Guzmán is 55, which in narco-years is about 150. When Pablo Escobar was Chapo’s age, he had been dead for more than a decade...