But for many others, the reason the deal failed was an emotional one. The agreement had always been a tug of war between peace and justice, and in the end, the demand for justice won.
“Sometimes, it even seemed like they knew they got off easy,” Diana Hurtado, 37, a dog walker in the capital, Bogotá, said of the rebels. “When you see photos from Havana, they are laughing.”
In some ways, Mr. Santos may have been a victim of the country’s success against the rebels. Before running for president, he served as defense minister, pursuing the FARC in every corner of the country and even into Ecuador, where he launched a raid that killed a rebel leader in 2008.
As president, Mr. Santos said that those successes, and his continued military campaigns against the FARC, had created the pressure that pushed the rebels to the negotiating table and kept them there.
So why, many Colombians demanded, did the government accept a deal in which most rank-and-file FARC fighters would be able to walk free? Even worse, they argued, was the fact that those who committed war crimes would face reduced sentences under the agreement.
Government negotiators repeatedly insisted that the rebels would leave the discussions before accepting harsher terms. The talks dragged on for four years in Havana. Even the transitional justice system in which FARC leaders would either be granted amnesty or given reduced sentences took 18 months to sell to the rebels.
Humberto de la Calle, Mr. Santos’s chief negotiator, said in an interview before the vote that it would be impossible to negotiate a deal to include prison time for FARC fighters, a demand made by many of the deal’s opponents.
Colombia Reacts to Vote
“It’s honestly the worst option — it’s not possible,” he said. “I don’t think the FARC will say, ‘O.K., no, I’ll accept prison.’ That’s a joke.”
Adam Isacson, an analyst at the Washington Office on Latin America, said the rebels had made concessions, too, including dropping longstanding demands for radical land and economic reforms as a condition for peace. They also agreed to a six-month timetable to abandon arms, despite concerns that they might be hunted by paramilitary groups.
“In the FARC’s view, they gave up a lot of ground,” Mr. Isacson said.
Pollsters struggled to explain why the result had defied the forecasts. César Valderrama, the director of Datexco, who had predicted a double-digit win for the agreement, said the “no” vote had risen in reaction to the celebratory signing ceremony that took place before Colombians went to the polls, an act of political stagecraft that apparently backfired on the government.
Many “yes” supporters also believed the referendum was coasting to victory, so they did not feel the need to vote, Mr. Valderrama said.
“They already felt like winners, and as winners, it wasn’t necessary to vote,” he said.
Even those who backed the “no” campaign said they were stunned by the result.
“I always said that the difference wasn’t going to be much, but I never thought we were going to win,” said Santiago Valencia González, a right-wing Colombian congressman representing the northern Antioquia region, which voted against the agreement.
Mr. Valencia said the Colombian right would begin pressuring the government to pursue peace on new terms. “I think it’s a great opportunity for Santos to listen to us,” he said in an interview in Medellín.
Some who voted for the deal said they liked the idea of peace a lot better than the specifics of the agreement. José Barbosa, the owner of a bakery in northern Bogotá, recalled fleeing a region of central Colombia because his neighbor’s sons were being recruited by rebels and he feared that the same would happen to his children.
But Mr. Barbosa, 59, said he had been pragmatic at the ballot box. “They got too many concessions, but the rebels aren’t going to give up arms if their backs are against the wall,” he said.
Voters like Ms. Solano felt differently.
When she was an infant, armed FARC fighters came to her family’s farm in a rural area and demanded food and other provisions. When her family refused, they were driven from their land, and the rebels stole their cattle. Years earlier, her grandfather had also been evicted by the rebels.
“My parents can’t talk about this without tears,” she said.
Ms. Solano eventually began studying graphic design here in Medellín, a right-wing stronghold of Álvaro Uribe, the influential former president who led the charge against the peace deal and described Mr. Santos, his former defense minister, as a “traitor” for granting amnesty to FARC fighters.
She now works as a cashier at a sprawling restaurant that overlooks the city from the mountains. Before the referendum, she took a 10-hour bus ride to cast her vote in her home region, Santander, where the FARC had twice evicted her family.
After voting on Sunday morning, the entire family, more than 50 people, gathered at her grandfather’s home in the city of Bucaramanga. As they discussed how they had voted, only one of them, Ms. Solano’s brother, said he had cast a ballot in favor of the peace deal.
After the gathering broke up, Ms. Solano took the long bus ride back to Medellín. She learned the outcome when her mother called her in the morning.
“I don’t think it’s a reason to make you happy,” she said. “I think it’s a reason to think about things.”
She said she was not sure what would happen next between the government and the rebels.
“We don’t know whether to be happy or to worry,” she said. “But we voted with conviction.”