Esteban Vanegas sang of his love for Colombia with a choir of fellow guerrillas, marking the signing of a peace treaty ending one of the world's longest wars. Just below the huge stage the Revolutionary Armed Forces of Colombia (FARC) improbably erected in the bush, his mother, Miriam, watched him sing, her face a mix of joy and pain.

Four days ago, she had been at home when television footage from the jungle conference showed Esteban among the thousands of fighters gathered here. He had been taken by rebels when he was 13 and she long feared he was dead.

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In photos: A look at Colombia's long civil war

She travelled six hours to the FARC assembly. Finally someone recognized the child in the faded photo she carried. Not long after, she had her arms around her lost boy, now a strapping fighter.

"It's indescribable – to see her after all these years," he said. For the past 36 hours, the two have moved through the muddy camp, hand-in-hand.

With reunions such as these, the peace deal signed Monday has started to reap its first fragile benefits.

In the days to come, FARC fighters will begin to assemble at a half-dozen gathering points around the country, to hand over their weapons and begin a long process of "reintegration," including psychological support, education (about 40 per cent are illiterate) and job training.

The deal was signed in Cartagena, a picturesque Caribbean port city on the other side of the country, in the presence of heads of state from Latin America, U.S. Secretary of State John Kerry and United Nations Secretary General Ban Ki-moon.

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President Juan Manuel Santos and the guerrilla's supreme commander, Rodrigo (Timochenko) Londono, signed the 297-page Havana Accord.

"The horrible night of violence that has covered us with its shadow for more than half a century is over," Mr. Santos said. Timochenko asked for forgiveness from FARC victims. "We are all prepared to disarm in our minds and our hearts," he said.

But here in the jungle, the FARC marked the event with a bash: Top Colombian bands made the 10-hour journey over rutted dirt roads from the nearest town with an airport, and thousands of local people, most of them subsistence farmers who are the FARC support base, came to dance all night.

The FARC has waged a civil war for 52 years, financed with narco-trafficking and infamous for kidnappings, attacks on political figures and the forced recruitment of children such as Mr. Vanegas, who is now 23.

The Vanegas family was displaced from a state to the north when he was a young child, after right-wing paramilitaries murdered his father – and then "disappeared" a nine-year-old brother who had witnessed another murder, saying they would come for the rest of the family unless they fled. Ms. Vanegas took her five surviving children through a series of moves, but lost another child to the paramilitaries.

And then when Esteban was 13, he went out to a guerrilla camp on a teenage lark and the guerrillas kept him. Ms. Vanegas appealed to authorities to try to bring him back. "They told me to stop bothering them. I'm sure if I were a person with money, they would have found my child," she said.

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Mr. Vanegas said he didn't know what to expect when he was first with the rebels, but adapted readily enough. He is now steeped in the language of the class struggle.

Through those years, he could never call home, for fear of endangering himself and his unit, and also his mother – people known to have heard from a rebel relative are often harassed by the military.

People told Ms. Vanegas to stop hoping she would find him, that he was almost certainly dead, killed in a government bombing. (In fact, in 2011 he was shot in the hip in a confrontation with the military, but recovered well after surgery in a FARC bush hospital.) But she stayed put in that town in case he ever came home. "My mother's instinct told me he was alive," she said.

Through those years of tragedy, as all of the cruellest aspects of Colombia's war were visited on her family, she said she would often ask her own mother, "Why me, why me? Why can't I have just one moment of joy?" That moment finally came this week.

With the signing of the Havana Accord, the rebels become a Marxist-Leninist political party, guaranteed a handful of seats in Congress, who will compete in the next election.

At the collection points for the guerrillas, fighters such as Mr. Vanegas will be guarded by the Colombian Armed Forces, the soldiers they have been fighting. Many of the fighters here have expressed concern that the government will not or cannot keep them safe – from paramilitaries, or drug-trafficking groups that will not be pleased with the FARC's pledge to tell all before a truth commission.

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Mr. Vanegas said he isn't worried, because the only people who know the pain of the war as well as the guerrillas are the soldiers they fought. (His mother, on the other hand, made clear with an extravagant raise of her eyebrows that she is very much doubtful the rebels will be well protected.)

The peace accord is not yet a done deal: Colombians will vote on whether to adopt it in a referendum on Oct. 2. "When, or whether, we see each other again depends on if the Colombian people say 'no' – if they do, it's a disaster," said Ms. Vanegas, who is 57.

Polls suggest the agreement will pass, and the government is committed to then rolling out an accelerated development program to bring schools, roads, infrastructure, agrarian and political participation to rural areas. Mr. Vanegas said that is what will really keep the country from returning to war. "War is lived by the poor people of the country – if there were guarantees that we would have a good life, this wouldn't have happened."

His mother has a different prescription. "For the peace process to work, we have to forgive, so peace can be a reality," she said. "If my [disappeared] children are dead, I will forgive the ones who killed them," she said, referring to the two other sons she has lost. And the FARC, who took her youngest son? "Yes, them too. For peace to exist we have to forgive."

These past few weeks, Mr. Venegas was writing a song about the peace accord for his rebel chorus to perform on Monday. "But when I saw my mom, all the words just went out of my head."

Editor's Note: An earlier version of this story incorrectly stated FARC supreme commander Timochenko's first name as Ricardo. In fact, it is Rodrigo. This version has been corrected.