Bodies dumped on the side of the road. Gun battles in upscale neighborhoods. Tractor-trailers set aflame on the highway. People plucked from their cars by armed men in broad daylight.

This is what it looks like when war breaks out within one of the most powerful criminal mafias in the world, the Sinaloa Cartel, pitting two rival factions against each other in a bloody struggle to control a multibillion-dollar narco empire.

The past few years had been relatively peaceful in Sinaloa state, in northwest Mexico, where the dominance of a single, cohesive criminal organization kept turf wars to a minimum, and official homicide rates were lower than in many major U.S. cities.

Then, in late July, came an unthinkable betrayal: Ismael “El Mayo” Zambada García, a godfather of the cartel, was tricked by the son of his former ally, abducted, forced onto a flight to the United States and arrested by American agents, according to U.S. officials.

Mr. Zambada described the treachery in a letter released by his lawyer, in which the drug lord said that on the day he was arrested, he’d been lured to a supposedly friendly meeting and then “ambushed” and “kidnapped” by one of the sons of his fellow cartel co-founder, Joaquín Guzmán Loera, known as El Chapo.