"We're facing a permanent invasion," Mr. Colom told radio listeners recently, as he discussed a state of siege he declared in late December in Alta Verapaz, a province just south of Mexico.

In Alta Verapaz, joint military and police patrols now roam the streets looking to capture members of the Zetas. The state of siege lets security forces search homes and make arrests without warrants. The siege has apparently borne some fruit—25 suspects are in custody and more than $1.2 million in illegal goods have been captured, including drugs, grenades and assault rifles, the government says.

While Guatemala is starting its own campaign against cartels, it hopes not to mimic Mexico in one key respect: Since the Mexican operation began in 2006, drug-related violence there has left more than 34,000 people dead.

Some people in Guatemala fear a surge in violence is exactly what will happen here, too.

"If we put our military against the military of the drug traffickers, we'll simply have thousands of deaths," said Sandino Asturias, a security expert at the Guatemala Studies Center, a think tank in the capital. Mr. Asturias recommends the use of intelligence like wiretapping to disrupt crime groups over the military's brute force.

Alberto Islas, a Mexican security analyst with a long history tracking his country's drug cartels, said Guatemala's military, trained to fight left-wing guerrillas during a 36-year civil war, "lacks the resources to fight organized crime," which he called "a different ballgame." Drug traffickers, for example, are much better funded than ideological groups fighting for a cause, Mr. Islas said.

That civil war, which left a legacy of brutal repression by the military and national police, led the government in recent years to slash the size of the army by 60%. Reductions and budget cuts meant that Guatemala could afford to send just 600 troops into the province to fight Los Zetas, compared with the nearly 6,500 soldiers that Mexico sent into the state of Michoacán to fight cartel La Familia, a cartel that at the time posed a similar threat.

In Alta Verapaz, the government ordered the closure of three military bases in 2004, dismissing nearly 11,000 soldiers there, and leaving the area virtually defenseless and badly exposed to organized crime, government officials say.

"I never imagined that the armed conflict had protected the country," President Colom said in a recent interview. "The guerrillas never got involved in drug trafficking. And then we reduced the military and the police."

While Central America has long been a stopping point for drugs smuggled from Colombia to the U.S., crime groups have stepped up their activities here in recent years, according to the U.S. authorities and drug experts.

In Guatemala, some $2.1 billion in drugs, cash and weapons were confiscated in the first half of last year, equivalent to about 5% of gross domestic product, and roughly the same as the amount captured for the whole of 2009, the government says.

And while it isn't known what prompted Los Zetas to come to Guatemala, some believe the timing may be tied to Mexico's own crackdown. Last month,, Mexican police announced the capture of a Los Zetas leader, Flavio Méndez Santiago, in the state of Oaxaca.

Los Zetas is among one of the most feared names in the drug wars. The group began after defectors from the Mexican Special Forces took up arms to defend the country's Gulf Cartel, only to break away last year into a criminal organization of their own that mixes drug trafficking with extortions, kidnappings and assassination.

The group made headlines in Guatemala in August when Mexican authorities blamed them for the bloody massacre of 72 would-be immigrants on a secluded ranch in Mexico. Many of the dead were Guatemalans headed to the U.S.

Residents of Alta Verapaz say Los Zetas began turning up three years ago. Valeriano Maquin, a 25-year-old man from the town of La Tinta, said the strangers introduced themselves as Guatemalan government officials and asked for local members of the Kaibiles, a former Guatemalan Special Forces unit believed to be responsible for civilian massacres during the country's civil war. Mr. Maquin and other residents believe the newcomers were looking to recruit the vigilantes into their own ranks.

Throughout 2008, Los Zetas fanned throughout the country, establishing a base of operations and confronting local drug traffickers for control of smuggling routes, Guatemalan officials say. In March 2008, 11 people were killed in a town in eastern Guatemala after a fierce battle between Los Zetas and a local gang.

Guatemalan Interior Minister Carlos Menocal said he was optimistic the government could control the crime groups including Los Zetas, pointing to past successes in the country's El Petén jungles, where airstrips used by drug traffickers were reclaimed in recent years.

But he conceded that the country's police force, meant to aid the military against drug traffickers, is also too small. Guatemala has just one officer for each 700 residents—compared with the one for every 400 recommended by the United Nations. The country has just 2,000 police investigators and needs 5,000, he said.

Some say there is more the U.S. government may do to help. In recent years, the U.S. gave Central American and Caribbean countries less than 25% of the $1.4 billion antidrug Merida Initiative, with the rest going to efforts in Mexico.

A State Department official said the government is increasing its focus on Guatemala along with El Salvador and Honduras and may be considering raising the funds if the countries show "political will to fund and address their domestic crime challenges."

Bringing the drug war to Guatemala faces opposition from leftist activist groups who say the use of soldiers to combat drug traffickers will lead to a remilitarization of Guatemala—and perhaps a return of the civil war in the country where some 200,000 were killed, mainly by the military.

Jorge Morales Toj, a former guerrilla who now is a lawyer and human-rights advocate, said military abuses such as rape and murder could return "the psychosis of war."

Mr. Morales and many others charge the military operation is political stagecraft, meant to draw more support Mr. Colom's government. Presidential elections are scheduled for September, and Mr. Colom's wife, Sandra Torres, is expected to make a run with Mr. Colom barred by term limits.

Mr. Colom denies politics have anything to do with the military operation and that no complaints of human-rights abuses in Alta Verapaz have been filed with the government since the state of siege began.

"Four people in my family were killed in the past conflict," he said. "I don't want to see this kind of thing happen again."