In “Wolf Boys,” Dan Slater writes about two teens from Laredo, Texas, who became assassins for the Zetas, a brutal cartel. Illustration by Keith Negley

After Gabriel Cardona was sentenced, in 2009, press photographers took his picture through a pane of protective glass, as if he were some exotic beast. There was something unthinkable about what he had become, a ghoulish contradiction too awful for the culture to assimilate: a child assassin. Yet there he sat, in pristine white prison scrubs, reciting a catalogue of macabre achievements in the matter-of-fact tones of a college interview. When Cardona was arrested, he was nineteen, and his delicate-featured face retained a dissonant boyishness. But he blinked when he spoke, in nervous flurries, and his interlocutors found themselves staring at a tattoo of a second set of eyes, blue-black and smudgy, that had been inked onto his eyelids.

In the past decade, as the death toll from Mexico’s drug war spiralled, it was all too easy for people in the United States to think of the horrors unfolding just across the border as a foreign problem, as disconnected from our day-to-day reality as the conflicts in Libya or Syria. But Gabriel Cardona was an American kid. Born and raised in Laredo, Texas, he was poor but smart, and fully attuned to the meritocratic ethos of life in the United States; as a child, he thought he might grow up to be a lawyer. Cardona played on the football team, read Buzz Bissinger’s “Friday Night Lights,” and identified with the stunted yearning of the characters in the book. Then, during his sophomore year, a coach benched him, and he ended up dropping out and drifting into delinquency—first stealing cars, later smuggling drugs and weapons across the border. As Cardona came of age as a petty criminal, a brash new cartel, the Zetas, was asserting itself in Mexico’s drug economy and developing a reputation for tactics of unparalleled cruelty.

Laredo’s population grew by nearly fifty per cent in the nineteen-nineties, as cross-border trade surged after the passage of the North American Free Trade Agreement, and the new relationship between Mexico and the United States transformed the underworld ecosystem as well. In a new book, “Wolf Boys: Two American Teenagers and Mexico’s Most Dangerous Drug Cartel” (Simon & Schuster), Dan Slater writes that by 2004 the Zetas were moving as much as ten tons of cocaine across the border—and grossing roughly a hundred million dollars—every week. They called their cartel the Company, and as that dirty revenue trickled into the sprawling metropolitan region that comprises Laredo and Nuevo Laredo, just across the border in Mexico, the area started to look like a company town. Small businesses became fronts for laundering drug proceeds, Slater writes, and “everyone, it seemed, was mixed up in something.”

Slater, a former reporter for the Wall Street Journal, read about the arrest of Cardona, and of his childhood friend and fellow teen hit man Rosalio Reta, in the press. In interviews with the Times and other outlets, Cardona and Reta described living in a Texas safe house and carrying out hits on demand. Slater wondered how an adolescent becomes a mass murderer. Cardona was seventeen when he joined the cartel and nineteen when he was captured. Reta, who, with his diminutive stature and oblong head, was known as Bart, after Bart Simpson, joined at sixteen and was in custody less than a year later. Between them, by their own accounts, they killed more than fifty people. Were they psychopaths to begin with? Or were they ordinary kids whom the Zetas had sculpted into monsters? Wanting to understand “the allure of cartel logic,” Slater wrote to Cardona and Reta in prison. To his surprise, they wrote back.

One day in the summer of 1995, a psychologist named Michael Wessells visited Grafton Camp, a rehabilitation center in Sierra Leone for child soldiers who had fought in the country’s civil war. The children ranged in age from nine to sixteen. Many of them had killed. But as Wessells watched they drew pictures and danced and played coöperative games. They behaved, in other words, like kids. In an essay, he recalled how he was struck, in that moment, by the realization that, “under certain conditions, practically any child could be changed into a killer.”

The phrase “child soldier” tends to conjure images of places like Sierra Leone, and minors were used extensively there and in other African conflicts during the nineteen-nineties. But boys and girls under the age of eighteen have been deployed in battles throughout the world, from Colombia to Sri Lanka, and still fight on the front lines of many conflicts today. According to the United Nations, recruitment of child soldiers in Afghanistan doubled last year, with both the Taliban and government forces relying on underage combatants. In March, the State Department reported that the Islamic State is increasing its dependence on a cadre of juvenile conscripts, some as young as ten years old, who are known as the Cubs of the Caliphate. Historically, children often served in ancillary roles during wartime, as couriers, drummer boys, or “powder monkeys,” who ferried ammunition to cannon crews. But as weapons design evolved during the past century, and particularly with the advent of the AK-47 assault rifle, it became more practical to put children in front-line combat. P. W. Singer, in his book “Children at War” (2005), observes that the AK-47, with fewer than ten moving parts, is “brutally simple”: “Interviews reveal that it generally takes children around thirty minutes to learn how to use one.”

What juveniles lack in strength and experience they make up for in other qualities: they are coachable and often available in abundant supply. The uncertainty of wartime leaves young people acutely vulnerable; separated from family or other support structures, children can form a dependency on their military commanders that makes them easy to exploit. The warlord Joseph Kony, in the early years of his insurgency in Uganda, conscripted adults for his Lord’s Resistance Army. He eventually switched to children, because they were easier to indoctrinate. Of course, there is a moral taboo associated with defiling the innocence of youth, but a willingness to violate that taboo can amount to a tactical advantage. A professional soldier, peering through the scope of his rifle at a twelve-year-old, might hesitate to pull the trigger. And signalling that there is no boundary one is unprepared to transgress may demoralize one’s adversary. A recent report by the Quilliam Foundation describes Islamic State propaganda videos that feature children committing murder, and suggests that the group is broadcasting its willingness to flout international norms in a deliberate effort to seize “the psychological upper hand.”

One context in which we don’t often hear about child soldiers is the drug war on the U.S.-Mexico border. Yet, according to Child Rights Network, an alliance of civic and social organizations in Mexico, some thirty thousand minors have been pressed into playing a role in the country’s ongoing criminal insurgency, and several thousand of them have been killed. “Wolf Boys” offers a bracingly intimate glimpse of how this insurgency looks from the point of view of the young killers on the front lines. Prison can make a good correspondent of almost anyone, and, after writing to Cardona and Reta, Slater found himself drawn into an epistolary relationship of queasy intensity. He visited both boys in prison and spoke to them for hours. Reta eventually cut off contact, but Slater and Cardona continued to correspond, exchanging hundreds of pages of letters.