Despite all the effort, there are many troubling indicators. Nationwide, the number of poppy-free provinces, which reached a high of 20 in 2010, has now dropped to at least 17 and could be found to be still lower once researchers finish surveying remote provinces. Overall acres under poppy cultivation began rising again in 2009 after a significant drop the year before, and the total has grown slowly but steadily since.

Interdiction, while somewhat improved under new Afghan counternarcotics leadership, nets only about 3.5 percent of the 375 tons of heroin that leaves the country every year, according to the United Nations.

Even the success stories are unlikely to be sustainable, officials say. The prime example is the combined American and British counternarcotics campaign in the Helmand River Valley, in the heart of the province that produces nearly half of Afghanistan’s opium. Since its start in 2009, the military mission has coincided with a 33 percent decrease in opium poppy cultivation in the area, and concurrent programs to create alternative jobs and crops have had a significant effect there.

But the troops are leaving — as many as 14,000 American Marines could depart Helmand by the end of the year — and many of the incentive programs are closing down unless Afghanistan’s counternarcotics minister can persuade the West to renew them.

“We have to watch the answer develop over the next 6 to 12 months,” Mr. Brownfield said, speaking of the effects of the military withdrawal. “That’s what transition is all about — we’re changing from a known to an unknown.”

This year’s low opium harvest has thrown another element of unpredictability into the picture. It has already driven a few farmers to commit suicide and others to flee because they feared retribution from creditors, according to the governor’s office in Helmand. But rather than serving as a disincentive, the poor crop is more likely to prompt many to plant even more poppy next year to make up for this year’s losses. That was the pattern in previous blight seasons, like 2010.