EMINENCE, MO. -- For more than three hours, Maj. Mike Hackenwerth had been buzzing his Missouri National Guard helicopter over the tranquil Ozark hills, frightening the horses and the cattle, while the men in his patrol scanned the terrain for their quarry.

By midafternoon, the search was starting to look hopeless when suddenly a Guardsman cried out: "Bingo!" Hands beckoned excitedly toward a small opening in the forest below and two state Highway Patrol pickup trucks zoomed off in the direction of the catch -- an emerald green island of budding marijuana plants.

"This is an adventure," Hackenwerth, a 43-year-old school teacher, said several hours later, after his National Guard-Highway Patrol team had yanked 389 pot plants from the Mark Twain National Forest. "We're playing mind games with these growers, and today we beat them."

The pot hunt in this remote region of southern Missouri is one front in what federal officials claim is the most ambitious and costliest campaign yet to eradicate the domestic marijuana crop -- a $45 million-plus operation that illustrates the expanding role the Pentagon is playing in the antidrug effort.

While public concern remains focused on crack cocaine and other hard drugs, the Bush administration has mobilized more than 3,500 National Guard troops and about 65 Vietnam-era Huey helicopters to support slash-and-burn marijuana raids by federal and state law enforcement agencies.

The Guard has been involved sporadically in antidrug operations since 1983, but never before in all 50 states and never with the kind of resources it has now been allocated. The operations have also included, for the first time, small numbers of regular Army troops, under a plan approved by Secretary of Defense Richard B. Cheney.

In the most dramatic example of that involvement, 200 federal agents and military troops, transported by two C-130 cargo planes and seven Army helicopters, descended on northern California's Humboldt County last month and sealed off 640 acres of the King Range National Conservation Area.

The idea of a new federal war on marijuana might at first glance seem puzzling. Federal surveys show that use of the drug has declined sharply and is now at its lowest levels in more than 15 years: The percentage of high school seniors reporting in annual surveys that they had smoked marijuana in the past month dropped by more than half -- from 37 percent to 17 percent -- between 1978 and last year.

Yet paradoxically, in large part because of increased federal interdiction efforts at the borders, the share of that market controlled by U.S. marijuana producers has been growing rapidly.

In the last five years, domestic marijuana production has more than doubled, ballooning into an estimated $20 billion-a-year industry, according to federal officials. Resourceful growers have defied the best efforts of federal agents to catch them, setting up sophisticated indoor greenhouses and moving their operations into federal forests, national parks and isolated rural communties like this one -- home of "Ozark Gold," reputedly among the most potent marijuana in the world.

The U.S. marijuana boom has disturbed federal antidrug officials for years, but more recently it has become a nagging foreign policy embarrassment for the Bush administration. A key point of the administration's so-called Andean antidrug strategy is to prod Latin American governments to adopt tougher measures, including the use of military force, to destroy their coca crops.

But increasingly, Latin American officials respond: Who are you to lecture us when your own marijuana crop grows larger every year?

"We can't be viewed as the Yankee hypocrites to the north," said Stanley Morris, deputy for "supply reduction" to national Drug Control Policy Director William J. Bennett. "We have to be as tough and aggressive about the dope being grown in the United States as we are asking other countries to be about theirs."

The California raid, dubbed "Operation Green Sweep," was a textbook example of what the administration has in mind. The anti-marijuana raiders included 58 soldiers, dressed in camouflage and equipped with M-16 rifles, from the U.S. Army's 7th Infantry Divsion at Ford Ord, Calif. -- some of them battle-scarred veterans of the Panama invasion. They were led by officials of the Interior Department's Bureau of Land Management.

But the operation provoked a storm of controversy. Demonstrators in tiny Garberville called the action "U.S. military terrorism." The local sheriff ridiculed the operation, noting that the federal raiders were barely able to find 1,000 plants -- less than 5 percent the number destroyed by routine law enforcement operations by local agencies this year.

"Some people are offended that our region is being treated like another Panama or something," said Rep. Douglas H. Bosco (D-Calif.). "And then some people think it's a joke -- I mean to have GI's crouching around in camouflage outfits in the woods and in people's gardens."

But administration officials staunchly defend the raid on the grounds that it was necessary to "reclaim" public land. "We've said that if we only took out one plant, it would be worth the effort," said Sheri Bell, chief BLM spokeswoman for Green Sweep.

Administration officials say similar operations will soon be launched in Kentucky, Oregon, Hawaii and possibly other states. In Kentucky, federal agents later this month will begin the large-scale herbicide spraying of marijuana, using a chemical called glyphosate, for the first time since the practice was stopped amid environmental controversy nearly a decade ago, officials said.

The battle here in Missouri, rated among the top five marijuana-producing states in the country, provide a case study in the expansion, and the pitfalls, of the government's anti-marijuana efforts.

Known as "Operation Cash Crop," the joint federal-state anti-marijuana campaign began here in 1983. It includes a toll-free, 24-hour hotline (1-800-BAD WEED) for citizen informants. Callers are encouraged to report "unusual or suspicious activity" such as a "house in the country where men are constantly going in and out" or an "unusual amount of vehicular traffic {especially at night} carrying unkown individuals on and off property," according to a state bulletin on the hotline.

The campaign also includes a contingent of 38 Missouri National Guard members who fly Highway Patrol officers on raids and accompany them as they march through fields and streams.

"This equates to small infantry insertions that you might do in military-type operations," said Maj. Ken MacNevin, Missouri Guard public affairs officer, explaining the Guard's enthusiasm for participating in the missions.

As Missouri officials see it, the Guard's involvement, which began in 1988, has been a boon to their efforts, helping to produce last year's record of 2,035,300 destroyed marijuana plants. But critics deride these figures as nonsense, contending that a large portion of the felled plants are simply "ditch weed" -- low-potency marijuana that has been growing wild in farm fields and roadsides across parts of the Midwest for more than a century.

"You might as well be trying to eradicate dandelions," said Dan Viets, a Columbia lawyer who defends accused marijuana growers and is also a board member of the National Organization for the Reform of Marijuana Laws. "Any botanist will tell you that wild pot is growing all over the state. . . . The whole thing is a ridiculous misappropriation of resources."

But here in the Ozarks, the state's most economically depressed region, the story is somewhat different. Once a haven for moonshiners, the lush green hills and humid climate of this region in recent years have yielded some of the richest, cultivated marijuana anywhere. "They call it 'Weed from the Land of Oz,' " said Terry, a 33-year-old carpenter with three children who said he has been growing marijuana off and on for nearly 10 years.

According to local officials, the marijuana boomlet here began in the mid-1970s when "back to the land" refugees from California, attracted by this region's remoteness, began moving in and growing small plots of marijuana. It did not take long for the "locals" -- woodcutters, farmers and others like Terry -- to catch on.

But Terry contends -- and most law enforcement officials agree -- that large-scale growers have left the area, driven away by the steady barrage of National Guard helicopter raids in recent years. "I used pot to supplement my income. . . . In a good year, I might have have made $6,000 to $7,000," he said. "But it takes a lot of nerve to do it with all these helicopters flying around. . . . Most people have given up."

Local law enforcement officials agree, suspecting that the big-time growers have migrated north into other regions of the state or south into Arkansas. "There's a lot of people that grow a few plants for themselves, but as far as as really big growers, I haven't found any this year," said Larry Rutledge, sheriff of Shannon County. "A lot of these people are on food stamps and welfare, and they're just growing a little dope."

Nevertheless, thanks to a 60 percent jump in the Missouri National Guard's antidrug budget and a nearly doubling in its authorized flying time, the campaign in the Ozarks has intensified. During one recent morning, a contingent of about a dozen Highway Patrol officers and Guardsmen, dressed in military fatigues, assembled at the main base at a local airport and then mapped out their attack plan: a strike at a nearby farm where an informant had reported rows of marijuana plants.

But an hour later, when Hackenwerth's Huey helicopter landed, the raiders discovered the suspected pot plants were actually rows of ragweed.

Regrouping after lunch, the patrol set out again -- this time, over to the next county. After hours of searching, they found six small gardens, some of them broken up into groups of three or four plants and then scattered about deep in the forest to mislead the agents.

While the total take was not large, Hackenwerth and the other Guardsmen were more than pleased. "It's kind of like when you been out hunting all day looking for deer or wild turkeys," he said. "You never want to come out skunkered."