On Oct. 19, 2021, Armando Linares López was writing up notes from an interview when his cellphone buzzed with an unknown number. Linares, 49 and stocky with black hair that was just starting to show gray streaks, ran an online news site in a small Mexican city called Zitácuaro. He knew his beat so intimately that calls from unfamiliar phone numbers were rare. But the man on the other end spoke in a way that was instantly familiar. Linares had come to know that pitched, menacing tone from years of run-ins with every kind of Mexican gangster. “This is Commander Eagle,” the voice said. “I’m from the Jalisco New Generation Cartel.” Zitácuaro, in the hills of the state of Michoacán, had for years mostly been known for its fertile avocado orchards and the pine-oak forest where tourists came to see the annual arrival of the monarch butterflies. But its central location had made it increasingly attractive to the drug trade. Farmers grew marijuana and opium poppy, the source of heroin, i...