To the surprise of many, this experiment has now led to a significant drop in coca plantings in Mr. Morales’s Bolivia, an accomplishment that has largely occurred without the murders and other violence that have become the bloody byproduct of American-led measures to control trafficking in Colombia, Mexico and other parts of the region.

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Yet there are also worrisome signs that such gains are being undercut as traffickers use more efficient methods to produce cocaine and outmaneuver Bolivian law enforcement to keep drugs flowing out of the country.

In one key sign of progress in Bolivia’s approach toward coca, the total acres planted with coca dropped 12 to 13 percent last year, according to separate reports by the United Nations Office on Drugs and Crime and the White House Office of National Drug Control Policy. At the same time, the Bolivian government stepped up efforts to rip out unauthorized coca plantings and reported an increase in seizures of cocaine and cocaine base.

“It’s fascinating to look at a country that kicked out the United States ambassador and the D.E.A., and the expectation on the part of the United States is that drug war efforts would fall apart,” said Kathryn Ledebur, director of the Andean Information Network, a Bolivian research group. Instead, she said, Bolivia’s approach is “showing results.”

Still, there is skepticism. “Our perspective is they’ve made real advances, and they’re a long way from where we’d like to see them,” said Larry Memmott, chargé d’affaires of the American Embassy in La Paz. “In terms of law enforcement, a lot remains to be done.”