There’s a troubling international dimension, too. Mr. Márquez says in his video that his group is recording from the Inírida River region, along the border with Venezuela. The rebooted guerrillas are likely to spend much time in Venezuela, where the Maduro government has been tolerant of, even aligned with, Colombia’s leftist bands. That raises the prospect of a regionalized, alarmingly escalated conflict.

It’s remarkable that it took this long for any top-level FARC leaders to defect, given the Colombian government’s slow, partial delivery on its peace accord commitments. Many of the government’s key promises laid out in the accord required codification as legislation. But to date , 57 percent of the laws necessary for the accord to be put in place are stuck in Colombia’s Congress, if they were even introduced.

Efforts to reintegrate FARC fighters into society and the economy have been chronically delayed. Today, while all are receiving a monthly stipend, only 4,000 of the 12,000 accounted-for guerrillas have found work or received promised assistance with agricultural or small-business projects. Agencies created to establish the accord’s rural provisions are to have their budgets cut by double-digit percentages in 2020.

The government has been unable to address a horrific wave of threats, attacks and murders of leaders of different societal groups. All around the country, associations of farmers, victims, coca-substitution program participants, village advisory boards, Afro-Colombian communities, and indigenous settlements are living in terror. At a time when local democracy should be flourishing, a local leader is murdered every two and a half days. Demobilized FARC members, too, have been victims of attacks and killings. The death toll is now between 126 and 132.

Ex-guerrillas also need certainty about their legal future. Mr. Duque and his party issue frequent verbal and legislative attacks on the post-conflict justice system (for example, against tribunals, whose structure took 19 months to negotiate, issuing light sentences for those who to confess war crimes). This has increased uncertainty among FARC members, fueling warnings from hard-liners like Mr. Márquez, that Mr. Duque’s government is bent on finding excuses to imprison or extradite them.

The peace process isn’t dead, but Colombia will need changes to avoid joining the list of countries that relapse into war within five years. The government must disprove the extremist FARC faction’s narrative.

Mr. Duque should immediately visit some of the 13 village-size zones where the guerrillas demobilized in 2017, and where a third of them still live today. There, he can listen to the former fighters’ concerns, and agree to break the bureaucratic and budgetary obstacles delaying livelihood assistance. Pending peace legislation must move through Colombia’s Congress with the government’s support. State presence needs to be felt by people who live in ungoverned rural areas. Mr. Duque needs to cease his verbal and legislative attacks, to alleviate the concern they have caused among former combatants.