Until recently, the U.S.-backed war on drugs seemed to be paying impressive dividends in the sparsely populated department of Putumayo in southern Colombia. Five years ago fully half of all the coca cultivated inside Colombia came from Putumayo, and guerrilla units from the left-wing Revolutionary Armed Forces of Colombia (FARC) were directly involved in the region's flourishing drug trade. But an aggressive, U.S.-funded aerial-fumigation program has helped slash coca farming in Putumayo by more than 90 percent since 2000, and some territory in the department previously under FARC control has been recaptured by the Colombian military.

The guerrillas, who continue to finance their military operations in part through cocaine trafficking, remain a formidable foe, however. Last month FARC bombed electrical pylons, destroyed a key bridge on the only road linking Putumayo to the rest of Colombia and declared an "armed stoppage" that forbade its 300,000 residents from traveling by road. Residents of Putumayo's largest city, Puerto Asis, were left without electricity or running water, and the Colombian Air Force had to fly in tons of food to avert looming shortages. The rebel blockade brought the region to a standstill for more than two weeks, giving authorities a fresh reminder of the guerrillas' firepower and resilience.

The recent paralysis in Putumayo is the latest in a series of troubling signs that the U.S.-backed war on drugs is faltering. The centerpiece of that effort is Plan Colombia, a multipronged counternarcotics initiative that was unveiled five years ago this month in the colonial port city of Cartagena with President Bill Clinton in attendance. Plan Colombia was designed to cut the country's narcotics production in half by this year, and the U.S. government has pumped $4 billion into the program thus far. During its first three years, U.S. and Colombian officials touted estimates showing a steady drop in the amount of land used for growing coca, from a peak of 169,800 hectares in 2001 to 113,850 two years ago. But for the first time since Plan Colombia's inception, the number of hectares planted with coca showed no decrease in 2004, according to U.S. State Department figures. Wholesale prices for cocaine in the United States have remained stable throughout the Plan Colombia era, even though seizures are up, belying government claims in both countries that the anti-drug program has reduced the quantities of cocaine being produced inside the country. "This is smoke and mirrors," says Bruce Bagley, a Colombia expert at the University of Miami. "The claims of success in reducing the overall flow of drugs from Colombia are highly exaggerated, and the profitability of the industry remains largely unimpaired."

Those harsh realities of the cocaine trade apparently pose no threat to the future of Plan Colombia, which is due to expire at the end of this year. Earlier this month President George W. Bush invited his Colombian counterpart Alvaro Uribe to his Texas ranch and pledged to seek additional funding from the U.S. Congress for the program. Senior officials of the Uribe government say they are very satisfied with the supposed achievements of the war on drugs, citing estimates compiled by the United Nations suggesting that the number of hectares planted with coca may be much lower than the State Department's figures. "I'd give Plan Colombia a '10'," says Colombia's Ambassador to the United States Luis Alberto Moreno. "Colombia has in fact reduced cocaine in excess of 50 percent, and from every measure you take, the situation shows progress."

But the track record of Plan Colombia is decidedly more mixed. Though originally billed as a counternarcotics initiative, the plan is a grab bag of policy objectives that include strengthening Colombia's security forces, reforming the judiciary, promoting human rights, substituting legal crops for coca and reasserting the Colombian state's presence in rural areas under the control of left-wing guerrillas and paramilitary warlords. Plan Colombia has claimed its greatest successes to date in areas not always related to the anti-drug effort. (When Bush listed the accomplishments of Plan Colombia this month at a joint press conference with Uribe, he highlighted a drop in violent crimes but made no specific mention of any narcotics matters.)

The lion's share of the U.S. aid earmarked for Plan Colombia has gone to the national police and the armed forces in the shape of helicopters, fixed-wing aircraft, other equipment and training programs. By most accounts, the assistance and training have bolstered the counterinsurgency capabilities of the military--which has seen the number of troops ready for combat rise by 60 percent since 1999--and extended the reach of the central government. "Without Plan Colombia, the police and military would not have the additional ability they now have to combat irregular [armed] groups and drug trafficking," says Alfredo Rangel, director of the Bogota-based Security and Democracy Foundation think tank. "The state has acquired a greater degree of control over territory in good measure thanks to Plan Colombia."

But despite all the bullish spin coming out of Bogota and Washington, Plan Colombia has failed in most categories of the war on drugs. The crackdown on coca farming in Putumayo had the effect of dispersing growers: the number of departments where the plant is cultivated has risen from a dozen five years ago to 20 today. The total amount of acreage planted with coca may be down nationwide, but the supply of cocaine clearly hasn't been disrupted: seizures jumped by 50 percent in 2004, and the purity of wholesale cocaine in the U.S. market, which gets 80 percent of its product from Colombia, has increased in recent years. The complete absence of a spike in the price of either cocaine or the coca base used to make the drug has dumbfounded U.S. officials. "This is a source of frustration," admitted U.S. Ambassador to Colombia William Wood in a guest column published in the news magazine Cambio last January. "There is much that we don't know about drug trafficking."

The law of diminishing returns may be undermining aerial fumigation, the main weapon in Plan Colombia's anti-drug arsenal. As coca fields become smaller and more scattered in response to the spraying campaign, the task of reaching those crops by air has become more costly and inefficient. The anti-narcotics police fumigated a record 136,555 hectares of coca in 2004, but the State Department actually reported a negligible rise in the total amount of acreage under cultivation. Some attribute the paradox to sharply increased crop yields and possible underestimates of the amount of land that is really being planted. "[Colombian officials] must feel frustrated because a lot of the places where they sprayed heavily increased [production] in 2004," notes Adam Isacson, head of the Colombia program at the Washington-based Center for International Policy research organization. "[The growers] will replant coca, which they've done time and time again."

Isacson and other critics say Plan Colombia should redirect the bulk of its resources toward promoting alternative economic-development schemes that give farmers incentives to abandon coca and building more infrastructure to improve their market access for legal crops like yucca and sugar. But that isn't likely to happen any time soon. The head of aerial-fumigation operations at the Colombian anti-narcotics police says he wants an additional six spraying planes and 10 more helicopters to go with the 17 fixed-wing aircraft and 26 helicopters already furnished under Plan Colombia. "We need more aircraft so we can fumigate those crops at least twice a year," says Lt. Col. Henry Gamboa. "We eliminate one harvest with a spraying operation, but each field of coca has the potential to produce at least four harvests each year." If the history of Plan Colombia is anything to go by, the men in uniform will be standing at the head of line when the next batch of U.S. anti-drug assistance is handed out.