SAN MARCOS, Guatemala — The convoy rolled out of the military base before dawn into the mist-shrouded mountains straddling Guatemala’s border with Mexico. Its mission: Destroy opium poppies used to make heroin.

Armed with rifles and machetes, the caravan’s nearly 300 soldiers and police officers from elite counternarcotics units scaled steep hillsides and waded through bone-chilling streams. They chased leads from drone pilots and inhaled dust as they rode in the back of pickups barreling down washboard dirt roads.

But after scouring village after village, they found only tiny plots of poppies here and there — a fraction of the region’s cultivation in previous years.

“The land here used to be covered in poppies,” Ludvin López, a police commander, said as soldiers fanned out around Ixchiguán, an area of remote hamlets populated by speakers of Mam, a Mayan language. But that was before opium prices plunged from $64 an ounce to about $9.60.

The largely fruitless search for opium poppies in Guatemala over several days in March laid bare a seismic shift in Latin America’s drug trade.

In the United States, the world’s largest market for illicit drugs, fentanyl has largely displaced heroin because of how cheaply and easily Mexican cartels can produce the synthetic opioid in makeshift labs using chemicals from China. Fentanyl is so potent that it can be smuggled in small quantities hidden in vehicles, another advantage over heroin.

As a result, demand for opium poppies has plunged.

In Guatemala, poppy farmers are losing their primary income from what had been their only cash crop, forcing many in poverty-stricken areas to migrate to the United States. At the same time, local and international authorities fear that Guatemala could emerge as a new hub for trading in the chemicals used to make fentanyl.

Drug busts along the U.S.-Mexico border also showcase heroin’s decline. In fiscal 2023, U.S. Customs and Border Protection’s Office of Field Operations seized 1,500 pounds of heroin, down from 5,400 pounds in 2021.

In the same period, about 27,000 pounds of fentanyl was seized, up from about 11,000 pounds.

Even as fentanyl lays waste to the heroin trade and counternarcotics priorities shift, U.S. authorities say U.S. support for poppy eradication efforts, though limited, is still needed in Guatemala to counter the reach of Mexican cartels that produce heroin.

Still, the highest priority in Guatemala is combating synthetic drugs and the detection of precursor chemicals used to make fentanyl, said a State Department official who was not authorized to be identified discussing drug interdiction strategies.

But the soldiers stomping through small vegetable gardens in remote villages were after opium poppies. Finding a few poppies, in patches no bigger than a hopscotch area, they went to work with machetes, chopping the plants. They did the same to the occasional cannabis plant, which remains illegal to grow in Guatemala.

Multiple signs of U.S. support for the mission — and for Guatemala’s counternarcotics efforts in general — were on display. Some police officers on the mission belonged to units supported by the Drug Enforcement Administration and undergo regular polygraph and drug testing. Soldiers traveled in four-wheel-drive vehicles donated by the U.S.

The State Department declined to provide a detailed breakdown of U.S. counternarcotics funding. But the country recently received $10 million to $20 million a year in military and police aid from the United States, according to Adam Isacson, the director of defense oversight at the Washington Office on Latin America, a research group.

That is roughly the same amount of such aid as a decade ago; overall, Guatemala ranks among the largest recipients of U.S. foreign assistance in Latin America.

An observer from the State Department, which has funded everything in Guatemala from training border police to an elite anti-gang unit, also accompanied the mission. He declined to comment, saying he wasn’t authorized to speak with journalists.

Since the soldiers’ efforts were mostly fruitless, they spent some of their time cracking jokes while mingling around their pickups. Trying to spread goodwill, some distributed items from their food packets to villagers; others gave away cheap plastic toys to children.

Still, in an exceptionally poor region where each mature opium poppy plant is worth about 25 quetzals (about $3.20), some villagers clearly seethed at the soldiers’ presence. Some refused to talk to anyone in the convoy, which they viewed as removing one of their only sources of income.

“We hardly have any poppies left around here anymore,” said Ana Leticia Morales, 26, a Mam-speaking mother of two who makes a living selling gasoline smuggled from Mexico. “But the soldiers still come, not to help us, but to make things worse.”

Mexican cartels relied on Guatemalan farmers to grow the poppies and then turn them into opium gum. Smuggled across the border into Mexico, the cartels would transform the gum into heroin.

The United States initially responded by spraying herbicides from planes in Guatemala but suspended those efforts after flight crews came under concentrated gunfire. This opened the way for the ground operations practiced today.

Fentanyl’s emergence over the past decade as a cheaper and much more profitable source of income for the cartels upended the poppy trade in Mexico while producing spillover effects in Central America. Now, the cartels don’t need to worry about heavy rains, which can destroy harvests. They also don’t need to worry about eradication operations.

Eradicators in Guatemala destroyed 2,011 acres of opium poppies in 2017 compared with 7 acres in 2023, Guatemalan government figures show.

The decline speaks to the ease in Mexico of using chemicals imported from China to produce fentanyl in labs about the size of a studio apartment, making it ideal for being manufactured in urban settings.

“It’s easier to produce a synthetic opioid in a laboratory than relying on a crop grown in remote mountains,” said Rigoberto Quemé, an anthropologist from the poppy-growing region of Guatemala. “The authorities are attacking the weakest link in the production chain,” a reference to eradication efforts. “But instead of disappearing, drug trafficking is still growing exponentially.”