As the U.S. prepares to send 30,000 more troops to Afghanistan on a mission that will include defending a civilian population in a narco-economy, Colombia's experience with drug traffickers and terrorism may be instructive.

The testimony of the former second in command of the 5th Front of the Revolutionary Armed Forces of Colombia (FARC), which operates in the banana-growing, drug-trafficking region known as Urabá, could serve as Lesson One.

Ex-guerrilla commander Daniel Sierra Martinez—known by the nom de guerre "Samir"—turned himself in to Colombian authorities in December 2008 in response to President Álvaro Uribe's national reconciliation offer. In exchange for a reduced prison sentence, he had to come clean about what he did in over two decades in the FARC. Last week Colombian authorities agreed to let him sit down with me and talk about his rebel experience.

Samir gave me an earful about the FARC's cocaine business and its exploitation of civilians in zones designated by "nongovernmental organizations" as "peace communities." He also told me that the supposed peaceniks who ran the local NGO were his allies and an important FARC tool in the effort to discredit the military.

In a September 2003 speech, President Uribe expressed his concern about the possibility that some "human rights" groups are actually fronts for terrorists. The international left, including Sen. Chris Dodd (D., Conn.), jumped all over the Colombian president for making that claim. But Mr. Uribe's comments were supported by information gathered by Colombian intelligence. Now, testimony from Samir, and countless others who have come in from the jungle, gives Mr. Uribe's charges further backing.

The 5th Front's territory includes a town called San José de Apartadó, which was designated as a peace community in the mid-1990s under a plan proposed by the local Catholic diocese. The idea was to create a place where civilians could live without fear of paramilitaries or guerrillas. As I reported in November 2003, "administration of this proposal, which pledged the full disarmament of all actors within the 'peace community,' was turned over primarily to Inter-congregational Justicia y Paz, a Colombian NGO. Justicia has had backing from Amnesty International and the Peace Brigades International."

But the peace community of San José de Apartadó, according to Samir, was not the least bit neutral. Rather, he says, the FARC had a close relationship with its leaders dating back to the early days.

Samir says that the peace community was a FARC safe haven for wounded and sick rebels and for storing medical supplies. He also says that suppliers to the FARC met with rebels in the town, where there were also always five or six members of the Peace Brigades International.

According to Samir, the peace community helped the FARC in its effort to tag the Colombian military as a violator of human rights. When the community was getting ready to accuse someone of a human-rights violation, Samir would organize the "witnesses" by ordering FARC members, posing as civilians, to give testimony.

Edward Lancheros, a member of the peace community council, and his cohorts (including a Jesuit priest called Javier Giraldo and Gloria Cuartas, the notoriously left-wing mayor of the municipality which includes San José de Apartadó), insisted that the "peace" required that the military stay out of the area. But the paramilitary was not about to observe such a convention. When clashes between the FARC and the paras occurred, Samir says, the peace community played a key role in shaping the story for the public to lay blame on the government.

One such incident occurred in 2000, when the paramilitary stopped an ambulance carrying a wounded female guerrilla and shot her. Samir says that the peace community claimed that she was a member of its group and alleged that the army had killed her. The community also helped hide FARC involvement in the area. In 2005, Samir says, a FARC rebel called "Alejandro" was killed by the paramilitary. The peace community insisted that he was a civilian from its group.

Samir says he resented the FARC's decision to take up drug trafficking and to work with drug-running paramilitaries. He also objected to the FARC's exploitation of the locals. Fed up with all this, he led a push back against FARC abuses and, in 2008, more than two dozen areas seceded from the peace community. He was then accused of being a military infiltrator and the FARC secretariat ordered him to be court-martialed. That's when he decided to turn himself in.

Of course his adversaries accuse him of making all this up to ingratiate himself with the government. But what cannot be denied is that while the FARC has been largely discredited among rural populations, it is the Colombian military, not the so-called peace community, that has pacified Urabá and given new life to its inhabitants.

O'Grady@wsj.com