Colombian President Juan Manuel Santos, left, and the leader of the FARC, Timochenko, congratulate each other on signing a historic peace agreement on Sept. 26. (Luis Acosta/AFP/Getty Images)

On a high school basketball court in a rough town surrounded by banana plantations, two FARC guerrilla commanders came to apologize for a massacre. Nothing like that had ever happened in Colombia until last week.

Sergio Jaramillo, the Colombian government’s peace commissioner, came for the ceremony, too, patting the shoulders of the victims’ relatives as they gave tearful accounts of lost husbands, fathers and sons. One of the relatives fainted. A few embraced the FARC leaders and wept.

Dressed in a white linen shirt, Jaramillo told the audience the time had come for Colombians to “confront their past” and begin making amends.

That the commanders of the Revolutionary Armed Forces of Colombia (FARC) would come seeking forgiveness in a place like this, where FARC gunmen killed 35 people at a party in 1994, was one of the objectives of the peace accord Jaramillo spent the past six years negotiating with the rebels.

But will it be enough? Jaramillo, as usual, looked worried.

On Sunday, Oct. 2, Colombian voters will decide whether to accept a peace deal with the FARC, whose members have waged the longest-running insurgency in the Western hHemisphere. The rebels met last week to discuss the accord and figure out their future after 52 years at war. (Nick Miroff,Jason Aldag/The Washington Post)

On Sunday, Colombian voters will go to the polls to approve or reject a pact with the FARC that would extinguish a half-century war. If the yes vote prevails, nearly 5,800 guerrilla fighters and thousands of additional FARC militia members will renounce their jungle hideouts and move into U.N.-monitored camps to lay down their weapons. The last major leftist insurgency in the Americas would be over.

It is more than an end of a conflict that has killed 220,000 people. Colombia’s peace accord promises to bury the country’s pernicious culture of political violence once and for all, and bring government services and development to the long-neglected rural areas where the FARC has thrived.

If voters reject the accord Sunday, there is no plan B. The deal would collapse. President Juan Manuel Santos has said the war would be back on. “It’s all or nothing,” Jaramillo said.

Opponents of the peace deal, led by powerful former president Álvaro Uribe, say that it is an unconscionable giveaway to a drug-
trafficking terrorist group and that its truth-and-reconciliation style justice tribunals will grant impunity to FARC war criminals.

“It’s not impunity,” Jaramillo counters. “Impunity is what we have now.”

The latest polls show the yes vote winning with 70 percent or more among likely voters. But Jaramillo is not that optimistic. The government’s own polling indicates that opposition to the peace accord may be stronger, he said. Low voter turnout is another worry.

A navy soldier paints the faces of a children with colors of the Colombian flag during a peace concert on Sept. 25, 2016. (Fernando Vergara/AP)

And with Santos struggling with a slowing economy and weak approval ratings — the FARC is even more widely disliked — there is a risk of a Brexit-style backlash Sunday that could sink the whole thing.

The government needs more than a narrow win. Because the peace accord commits the Colombian state to invest billions of dollars in long-term rural development, among other costly measures, Jaramillo is hoping for a victory margin large enough to look like a clear mandate.

This was always the risk of submitting a complex and politically fragile peace accord to a voter referendum.

“We believed an ambitious agreement like this could only succeed if it had democratic legitimacy,” Jaramillo said, “and if people thought that they had a stake in it.”

Jaramillo, 49, a former defense official with graduate degrees from Oxford, Cambridge and the University of Toronto, is often described as the “architect” of the peace accord. The label isn’t quite right, he said. “If I were the architect, I could have sent in the blueprint and gone home, instead of working until 11 p.m. every night.”

He opened secret talks with the FARC in 2011 that were made public in the fall of 2012 once the government and the rebels agreed on a basic framework. Santos appointed a widely respected lawyer and former politician, Humberto de la Calle, as the chief negotiator.

Jaramillo, the other principal negotiator for the government, was the chess master of the process, with the difficult task of building an agreement that would be acceptable to both the FARC and, eventually, to Colombian voters.

Unlike the previous attempts to negotiate with the Marxist guerrillas, the government this time made clear that Colombia’s political and economic model was not up for debate. The rebels wanted to repeal Colombia’s free-trade agreements; Jaramillo said it was a non-starter. But the two sides found common ground on the promise of rural development, whose biggest impediment, from Jaramillo’s perspective, was the war itself.

The FARC wanted to negotiate in Colombia or Venezuela, but Jaramillo and the government team preferred Cuba. It was relatively isolated from Colombian politics and media pressure. The FARC leaders felt safe. The International Red Cross helped pluck them from the jungle and deliver them to Havana.

There were divisions among the FARC commanders, many of whom hadn’t seen or spoken to each other in years, Jaramillo said. But Jaramillo said he developed working relationships with key FARC commanders such as Pablo Catatumbo and Pastor Alape.

During one potential breaking point, after FARC guerrillas ambushed and killed 11 Colombian soldiers in April 2015, Jaramillo met with Catatumbo at Santy’s, a ramshackle ceviche and sushi joint on the outskirts of Havana, to put the talks back on track.

The two countries that were critical to the peace deal, ironically, were the United States and Cuba, the old ideological foes who fought proxy battles through Latin American guerrilla groups like the FARC during the Cold War.

“You can think whatever you like about the Cuban government,” Jaramillo said. “But Cuba behaved generously and extremely professionally, and they were critical in helping sort out the most difficult moments. This peace process wouldn’t have worked anywhere else.”

Jaramillo said he met with U.S. Secretary of State John F. Kerry for a breakfast in September 2014 and asked for the Americans to directly support the negotiations. Within weeks, the Obama administration had named U.S. special envoy Bernard Aronson, a diplomat experienced in Latin America who both sides later praised effusively for his mediation contributions.

Another breakthrough came during President Obama’s historic Cuba visit in March, Jaramillo said, when Kerry went to meet privately with top FARC commander Rodrigo Londoño (alias Timochenko). Kerry assured him the United States would help the Colombian government go after the drug-trafficking groups that FARC leaders viewed as their biggest threat. It made a big impression on the FARC, Jaramillo said. “Kerry deserves a lot of credit.”

Jaramillo and the government will find out Sunday if the deal they made with the FARC was the right one, and if Colombians are ready to forgive.

Asked whether he thought the Colombian negotiators might win the Nobel Peace Prize, which will be announced Oct. 7, Jaramillo shook his head and declined to answer. He wasn’t looking that far ahead. He was only worrying about Sunday’s vote.