The men in thatched huts, lined with a dozen microwave ovens drying coca leaves, feigned the business of making cocaine. Outside, in a silent tightening circle, 45 select Colombian army students staging a practice drug bust approached the huts from the ridge above a creek.

"This is the most crucial moment for us," said Gen. Mario Montoya, head of Colombia's Joint Task Force South, the chief recipient of an enormous U.S. aid package. "They are within 20 meters of us now. And you cannot see them or hear them."

Suddenly, a shout from the trees: "We are the anti-narcotics battalion! You are completely surrounded!"

One of two people acting as armed guards, dressed in the rubber boots and olive uniform of leftist guerrillas, opened fire with blanks. And a column of men, faces streaked with green and black paint, rushed toward the collection of huts arranged in a likeness of the real cocaine processing labs that fill these southern jungles.

Once the area had been secured, a member of Colombia's attorney general's office reviewed the "captured" drug traffickers, along with plastic bags full of ersatz cocaine. Soldiers hurried away a "wounded" man to a waiting helicopter.

"There were some problems," Montoya said after the exercise as a host of U.S. officers in green berets and floppy camouflage hats looked on. "You don't group the captured together. And there were too many people inside the objective. Some should have stayed on the periphery. . . . But this is as realistic as it gets."

This staged "takedown" of a cocaine lab was part of a lesson plan designed by the U.S. Army 7th Special Forces Group training Colombia's new anti-drug battalions in the art of war on this vast base 240 miles south of Bogota. The class is almost over for the current batch of 728 soldiers, who this month will become the final battalion to finish the course and enter the intensifying fight against Colombia's assorted illegal armed groups that dominate the drug trade in the south.

Taken together, the 3,000 members of the four-battalion anti-drug brigade are the human spearhead of Plan Colombia, supported by a $1.3 billion U.S. aid package designed to attack a drug trade here that accounts for almost 90 percent of the world's cocaine supply. Since December, the first two field battalions totaling 1,500 men have been used mainly as ground support for an intensive aerial herbicide spraying campaign over two southern provinces. A handful of soldiers have been killed, including one Wednesday night in a clash that also left seven guerrillas dead.

Colombian officials say that more than 60,000 acres of coca, perhaps as much as a fifth of the country's crop, have been destroyed by the spraying campaign that relies on U.S. intelligence for targeting. With the last battalion's graduation and the impending arrival of U.S.-donated Black Hawk helicopters, the drug war here is about to pick up pace and shift slightly in focus to a more ground-oriented assault on the labs that turn leaves into drugs, bringing the soldiers into closer contact with the armed groups that guard them.

"They realized they had a unique opportunity to spray a lot of coca at one time," said a U.S. military official here, explaining why the plan's most controversial element has been the chief strategy so far.

To prepare for the social investment portion of Plan Colombia -- an effort to help farmers abandon coca growing and turn to legal crops -- the official said the Colombian military must start securing coca zones from the armed groups that control them.

Most of the world's cocaine originates here in southern Colombia as coca leaf; it ends up on the streets of U.S. cities in its processed powder form. Much of the money from the sale of the drug fuels Colombia's civil conflict, as armed groups of both the left and right profit by protecting the coca fields, clandestine airstrips and processing labs. Their role blurs the distinction U.S. policymakers have tried to make between fighting drugs and fighting Colombia's decades-old leftist insurgency.

This base of rolling pastures and jungle-covered knolls was once the hacienda of a powerful local farmer, killed decades ago by the Revolutionary Armed Forces of Colombia, or FARC, the country's largest rebel group. It sits on more than 20,000 acres 65 miles southwest of San Vicente del Caguan, the main city within a demilitarized zone that President Andres Pastrana turned over to the FARC two years ago for peace talks.

Since April 1999, the base has served as the main U.S. training center for Colombia's anti-drug battalions. Almost 90 U.S. military advisers, about half of them trainers, work here with the Colombian troops. The course includes topics from escaping a jungle ambush to administering first aid and maintaining a tricky battlefield respect for human rights.

The 7th Special Forces Group, based at Fort Bragg, N.C., added human rights training to the curriculum after some of its former clients -- the Atlacatl Battalion in El Salvador, for example -- were accused of massive rights violations during the 1980s. Now human rights training, mostly taught through hypothetical scenarios and role-playing, is woven into just about every element of the 18-week course. To be selected, a soldier must be vetted by the U.S. Embassy to ensure that he has a clean service record.

Montoya, a bluff general with a slightly graying crew cut, said: "We are teaching the value of human life here. These are narco-traffickers, but they are not enemies. When a man surrenders, he surrenders."

The fight here in the south would not exist without U.S. military hardware and advice. Montoya, whose task force consists of 11,000 men, including the three "line" anti-drug battalions and one support battalion, acknowledges that he would have "zero" helicopters without Plan Colombia. And in a remote jungle region where the only possible travel is by air or river, the helicopter is the transport of choice.

Already 33 U.S.-donated UH-1N transport helicopters have arrived as part of the package, with 16 of the larger, swifter UH-60L, or Black Hawks, scheduled to begin arriving in July. U.S. advisers here say that the Black Hawks, which can carry twice the number of troops as the helicopters currently in use, will significantly intensify the tempo of anti-drug operations by allowing the three battalions to conduct simultaneous strikes against labs.

From a hilltop here, with groves of palms and hilly pastures stretching into the distance, a U.S. military instructor looked down on a grassy clearing where 22 U.S.-donated helicopters sat in long, straight lines. The sergeant pointed to an adjacent pasture where a 10-man patrol moved slowly in a wedge formation toward a cluster of bushes. "Actually there are cows in the way now," the instructor said, shrugging as he explained the exercise.

Suddenly, rifle fire erupted and the patrol hit the ground. A grenade exploded, which the instructor said was a signal for the members of the patrol to "lift and shift" their fire to avoid hitting each other as they retreated from the ambush. The cows scattered.

"Now each man will give an ACE report -- ammunition, communication, equipment," the instructor explained. "That will determine whether they can go on."

In a month, the drill will become reality. Meanwhile, Colombia's drug-financed conflict is picking up in intensity. Montoya said that, after years of profiting from the drug trade, the FARC has accumulated a financial buffer that will take months to chip away.

"They are not suffering yet," Montoya said. "But if we continue at this pace, they will feel the effects in a year or a year and a half."

A U.S. soldier helps train a Colombian anti-drug battalion at the Larandia military base, 240 miles south of Bogota.Colombian soldiers practiced maneuvers earlier this month at their base.Colombian Gen. Mario Montoya leads a task force of 11,000 troops.