The journey through Mexico has always been dangerous for undocumented Central American migrants. Traveling without legal visas or any connection to the local population, they risk robbery, rape, and murder. But now the dangers are almost impossible to escape. As Martínez follows migrants up the eastern side of Mexico, he keeps encountering towns that are ruled by Los Zetas. “We realized,” Martínez writes, “that machetes had given way to assault rifles; that remote mountaintops had given way to safe houses; that your everyday delinquents had joined Los Zetas; that robbery had turned into assaults and abductions.” Consolidation may work for Mexico’s government and our own Drug Enforcement Agency, but for average Mexicans and desperate migrants it’s a disaster.

Every year cartels kidnap thousands of migrants and hold them for ransom. In 2007, a cartel kidnapped 300 Mexican and Central American migrants at once in Sonora. A priest managed to negotiate the release of 120. “Most of them were beaten black and blue,” Martínez writes, “and had their ankles broken by a bat.” Afterward not one filed a police report and the remaining 180 victims were never heard from again. For cartels like Los Zetas, kidnapping is an easy franchise: They delegate the violence to local gangs, supply them with guns, give them permission to use the name “Zeta,” and collect between $300 and $1,500 for each migrant. They can sell the women to brothels.

Given these circumstances, it’s hard to understand why anyone would attempt the journey north—until you realize that many of the migrants are already facing murder or destitution at home. In the first chapter of The Beast, Martínez introduces three brothers who fled El Salvador after they were threatened with death—they had asked too many questions about their mother’s murder. Another migrant used to work as a police officer in Honduras. After her first and second husbands were killed by members of the Mara Salvatruchas gang, she fantasized about shooting herself, her daughters, and her dog. “Then we’ll have nothing left to fear,” she thought. In the end, she decided to try for the US instead.

At times, Martínez seems almost angry that the US doesn’t make it easier for migrants like these to enter our country. And certainly he’s correct that no amount of border tightening will deter migrants fleeing terror, any more than the threat of incarceration deters drug traffickers from pursuing criminal profits. Recently, President Enrique Peña Nieto has redoubled Mexico’s efforts to arrest cartel leaders; the notoriously sadistic Zeta leader Miguel Angel Trevino Morales was captured in July. Such headline-grabbing arrests might play well in the international press, but if history is any guide they won’t hurt cartel operations. Extradition or even death simply spur institutional reorganization. A single kilo of cocaine, purchased in Colombia and sold in New York, brings more than $20,000 in profits. Nobody is going to let a business like that die just because the top man goes down.

In her book, Hernández presents a better strategy: Attack the money. In Mexico, the popular wisdom about cartels is that they live by “plata o plomo.” Silver or lead. Benjamins or bullets. If the narcos target you, you pay or you die. At the heart of this saying is a profound understanding of the narco psychology: what motivates them is money. They are the ultimate cutthroat capitalists.

To make her point, Hernández quotes extensively from Edgardo Buscaglia, who now works as a senior researcher at Columbia Law School. “I always tell friends and colleagues that El Chapo’s goal is to pay taxes,” says Buscaglia. “When the criminal groups legalize their wealth, their success is complete.” Seizing their cash, he argues, is the only way to truly disrupt their logistics and their power to corrupt. But weaning Mexicans from the billions of dollars in drug profits that have propped up their legal economy for more than a decade? That sounds as easy as weaning Americans from the more than 100,000 kilos of cocaine they consume each year.

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Marcela Valdes is a freelance writer who specializes in Latin American literature and culture.