IN COBAN, GUATEMALA The once-fearsome Guatemalan army has returned to the jungles where it battled Marxist guerrillas a generation ago, this time to hunt shadowy Mexican drug traffickers fighting for control of strategic smuggling routes to the United States.

So serious is the perceived threat to national security that Guatemalan President Alvaro Colom has extended "a state of siege" and martial law in his country's Alta Verapaz province, where authorities say the soldiers are beating back an invasion by the Mexican cartel known as the Zetas.

Colom plans to extend troop deployments to other conflict zones in the country, officials say, militarizing the drug war here and reviving the Guatemalan army after 36 years of civil war, decades of human rights abuses and a still-unfinished peace process.

The military operations are the clearest sign yet that as Mexico's wealthy drug mafias spread into Central America, wary but weak governments here are preparing to follow Mexican President Felipe Calderon's U.S.-backed decision to turn the armed forces against the cartels. That strategy has failed to slow the violence in Mexico, which has left more than 34,000 dead in four years.

The move is also likely to renew calls for more U.S. anti-narcotics aid beyond the $250 million allocated to Central America since 2007 as part of the $1.8 billion Merida Initiative, which has mostly targeted Mexico.

Last month, Colom made a remarkable call for a unified counternarcotics force that would set aside nationalist rivalries to combine soldiers from Guatemala, El Salvador and Honduras to retake territory from the expanding crime syndicates.

The Guatemalan president is also asking for help - more money, training, equipment and intelligence - from Mexico and the United States. The U.S. government recently moved helicopters from Colombia to Guatemala to expand operations by the Drug Enforcement Administration.

In the past year, DEA operatives working in Guatemala have sharply curtailed narcotics shipments into the region by air, as armed U.S. agents riding in Huey helicopters have chased down and ambushed smugglers when they attempt to land at clandestine airstrips. The number of known drug shipments arriving in Guatemala by light aircraft dropped from more than 50 to five or six in 2010, according to a U.S. official.

But because of that success, land routes through Guatemala have become even more critical to controlling the flow of narcotics, specifically shipments of South American cocaine bound for the United States, the world's biggest consumer of illegal drugs. The city of Coban and the surrounding Alta Verapaz region sit at the crossroads of many of those smuggling corridors.

U.S. officials say feeble institutions and venal governments make Central America particularly vulnerable to the cartels' wealth and power. In recent years, two of Guatemala's former national police directors have been arrested on corruption charges, along with former president Alfonso Portillo, now on trial facing allegations that he embezzled $15 million.

Cartel unchecked

Before soldiers arrived in Coban in December, the people of this farming town of 80,000 say they were powerless to confront the rapacious outsiders who rode through the streets in shiny new SUVs, brandishing automatic weapons and looking to recruit local talent with fat wads of dollar bills, especially former Guatemalan special-forces soldiers.

"The people here were too scared to do anything," said Col. Marco Tulio Vasquez Sanchez, commander of the 300 troops now patrolling Coban and the surrounding area.

At his desk in the military barracks, the colonel opened his laptop and played a video that soldiers confiscated at a farm allegedly abandoned by retreating Zeta forces in December. The disc shows a Sept. 15 Mexican Independence Day celebration at the local airstrip, where beefy men wearing white cowboy hats with pistols on their hips cheer horse races from the backs of their pickup trucks. The soundtrack is a bouncy Mexican ballad known as a "narcocorrido."

"That's the one they call 'El W,' " the colonel said, zooming in on a mustachioed figure in dark aviator glasses. "I'm still hunting for him."

Military commanders say they have seized dozens of vehicles, many of them stolen from Mexico, along with five airplanes and more than 100 weapons, including grenade launchers and machine guns.

Many of the arms were recovered at a farm on the outskirts of Coban, where they had been hastily buried in grain sacks beneath towering palms and plants heavy with coffee beans.

In a muddy grave behind the farmhouse, troops unearthed the body of a man kidnapped near Guatemala City.

According to the military, the farm belonged to Otoniel Turcios, a suspected Guatemalan drug smuggler who fled to Belize, where he was arrested in October. He was then handed over to DEA agents and flown to New York, where he is charged with conspiracy to import and distribute narcotics.

U.S. law enforcement agents say Turcios and other alleged local drug bosses, including Walter Overdick, known as "El Tigre," may have brought the Zetas into the country as partners or protectors but were quickly muscled aside.

"They invited the Zetas to the party, and the Zetas decided to take over," said a senior U.S. law enforcement official who spoke on the condition of anonymity because of his work in the region and security policies.

A mysterious text message purportedly sent by the Zetas to local journalists in late December was read aloud on radio stations in Guatemala. The message accused the president of taking millions in bribes but failing to keep his promises.

"A war will begin in this country, in shopping centers, schools, police stations," the announcement went.

Unlike in Mexico, where cartel gunmen stage brazen assaults on government officials and security forces, the criminals in Coban have not followed through on the threats of attack. Instead, since the military's arrival, the Mexican traffickers here have been mostly ghosts, the subject of sightings, rumors and anonymous Internet postings but very few arrests.

None of the 21 suspects taken into custody since December in Coban is a Mexican national, and authorities said the Zetas commanders have probably slipped back into Mexico or relocated to more lawless parts of Guatemala.

"This is just a slight disruption for them," said one U.S. official who spoke on the condition of anonymity, citing security protocols. He estimated that 70 percent of Mexico's 550-mile border with Guatemala is controlled by the Zetas.

Growing presence

Mexican traffickers have been caught in Guatemala, though, including two alleged members of the cartel La Familia arrested Jan. 29. In September, a Zetas commander, Daniel "El Cachetes" ("Cheeks") Perez Rojas, was convicted along with more than a dozen underlings for crimes linked to the massacre of 11 local competitors. And the boss of all bosses, the billionaire Sinaloa drug lord Joaquin "El Chapo" ("Shorty") Guzman, was seized in Guatemala in 1999, only to escape in a laundry basket from a Mexican prison two years later.

But the Mexican cartel presence in Guatemala has never been as extensive as it is now, according to Guatemalan officials.

"They say to the families, 'You can sell to us and leave standing up, or you can refuse and go feet first,' " said Oscar Pop, an indigenous leader in the region.

Pop said the return of the soldiers has stirred conflicted feelings among those who suffered during Guatemala's long civil conflict. An estimated 200,000 people were killed in the war, which ended in 1996.

"We need to be strengthening the police, not bringing back the army," he said.

The governor of Alta Verapaz, Jose Adrian Lopez, said he has been surprised that the military presence has been so welcomed, given the army's brutal legacy in the region.

"Now all the local mayors here are asking the government to send troops," he said.