An announcement by Néstor Humberto Martínez, Colombia’s attorney general, earlier this month reveals the sad state of that country’s November 2016 “peace agreement” with the narco-terrorist Revolutionary Armed Forces of Colombia. Mr. Martínez said that he has evidence that there are officials inside the agreement’s “special peace court” who are “cooking up falsehoods and fraudulent processes” in violation of the Colombian Constitution.

Mr. Martínez pledged to prosecute these “unscrupulous” individuals for hiding information about members of the guerrilla group, known by the Spanish acronym FARC, who have violated the terms of the agreement requiring them to report to special enclaves and confess their crimes. This fits with reporting from the region that thousands of renegade FARC members have rearmed and returned to the battlefield. Their goal is to reclaim their turf from other drug-trafficking armies.

Welcome to Colombia, where the much-ballyhooed deal, crafted in Havana by the government of then-President Juan Manuel Santos and FARC leadership, is unraveling faster than you can say “crime pays.”

The scandal at the peace court is only a slice of the trouble left behind by Mr. Santos, who handed power to President Iván Duque in August. As Bloomberg reported Sept. 11, Colombia is now experiencing a spike in rural violence, which had been greatly diminished under President Álvaro Uribe (2002-10).

Credit for this burgeoning regional security disaster also goes to former Secretary of State John Kerry, who backed the plan, and President Obama’s envoy to the Havana talks, Bernard Aronson. They took their bows and have scampered off. Rural Colombians are left with the bloody mess.

The collapse of the deal was predictable. The FARC is an organized-crime network that traffics cocaine, runs kidnapping and extortion operations, and engages in illegal mining. It got into these activities as far back as the 1980s, and while political power remains its ostensible goal, the objective has long been to keep and expand the businesses. It was foolish to think that a democratic state could find middle ground with these gangsters.

Mr. Santos promised he wouldn’t strike a deal unless it sent FARC leaders to prison for their crimes against humanity. He also pledged that the top commanders would have to turn over their drug-trafficking profits to compensate their victims.

FARC leadership refused those terms. But Mr. Santos was so hungry for something to distinguish his presidency that he broke his word and agreed to what is essentially an amnesty for capos.

They were supposed to surrender to special “zones” of reincorporation, where they would be confined for a time and tell the peace court about their atrocities. Norway was so impressed by the agreement that it awarded Mr. Santos a Nobel Peace Prize.

FARC leaders never accepted restraints on their liberty. They did no penance in the countryside. Instead they returned from Cuba to take victory laps around the nation, lecturing civilians about the beauty of Venezuela-style socialism. They demanded unelected seats in Congress, and Mr. Santos gave them, violating a promise not to do so.

They didn’t renounce their lives as mobsters. In April federal prosecutors in the Southern District of New York charged “FARC members and associates” with “conspiring to import” 10 tons of cocaine into the U.S. The accused include Jesús Santrich, one of the FARC negotiators in Havana, and Marlon Marín, the nephew of Iván Márquez, another Havana negotiator for the FARC. Messrs. Santrich and Márquez had been slated to take unelected seats in Congress.

Mr. Marín surrendered to authorities and was extradited to the U.S. with his family, where he has likely agreed to cooperate in exchange for witness protection. His uncle Iván has gone underground, giving up his congressional seat. Mr. Santrich was arrested and is fighting extradition to the U.S.

FARC apologists say prosecution would violate the Havana agreement. But the Southern District charges specify that the crimes were committed between June 2017 and April 2018. If proven, it would mean that the FARC members maintained their drug businesses in violation of the deal.

Many FARC rank and file also haven’t gone straight. Some immediately joined the National Liberation Army, a cousin to the FARC. Many others went to the special camps but have since fled. Some of those have done so, as Attorney General Martínez suggests, with the help of insiders at the peace court.

FARC criminals should be in prison. Instead they received lump-sum cash payments to launch new lives and they get monthly stipends that are far more generous than military grunts get. The FARC claims that the government camps aren’t up to their standards. It’s true. They had asked for gymnasiums, AstroTurf soccer fields and tile floors in the bathrooms. They didn’t appreciate having to work to build their housing.

Mr. Santos’s vision of converting the FARC into forest rangers and eco-tourism guides isn’t working out. Maybe that’s because, to paraphrase Willie Sutton, cocaine is where the money is.

Write to O’Grady@wsj.com.