The FARC was pushed back, government teams ripped up vast plantings of coca, and farmers planted cacao and other substitute crops. The region became a showcase. American members of Congress, business executives, diplomats, foreign correspondents and many others were brought in to get a glimpse of the future of Colombia.

The project set out to bring rural residents under the governmental umbrella for the first time and create the conditions for them to succeed. That meant improving roads needed to bring crops to market, giving titles to farmland, granting agricultural loans, bringing electricity to isolated hamlets, and setting up government services like courts, schools and health clinics.

But critics say that after Mr. Santos was elected president in 2010, the government’s interest seemed to wane.

Álvaro Balcázar, the original director of the program, who left in 2012, said that it suffered from the start from bureaucratic inertia and that other government agencies were slow to get involved. After some initial success in La Macarena, the program was expanded to include other areas of the country, but he said it ultimately lost focus.

“The promise was that the government was going to establish a presence in the regions, and that promise has not been met,” Mr. Balcázar said.

The area around Vista Hermosa shows notable achievements and stunning shortfalls.

This was once one of the country’s most productive coca-growing areas. It was also part of a zone that was ceded to the FARC, which uses drug trafficking to finance its activities, during an earlier round of peace negotiations from 1999 to 2002.

Today, fields once bright with the pale green of coca plants are now filled with the dark green of cacao bushes planted with American assistance.