Colombia's government and its biggest rebel group announced a deal Wednesday evening for ending their country's half-century guerrilla war, one of the world's longest-running armed conflicts.
The government's accord with the Revolutionary Armed Forces of Colombia must still be ratified by voters in a plebiscite in order to take effect.
But the announcement in Havana of a deal after four years of talks opens the possibility for Colombians to put behind them political bloodshed that has claimed more than 220,000 lives and driven more than 5 million people from their homes.
The accord commits Colombia's government to carrying out aggressive land reform, overhauling its anti-narcotics strategy and greatly expanding the state into traditionally neglected areas of the country.
Negotiations began in November 2012 and were plagued by distrust built up during decades of war propaganda on both sides.
Polls say most Colombians loathe the rebel group known as the FARC and show no hesitation labeling them "narco-terrorists" for their heavy involvement in Colombia's cocaine trade, an association for which members of the group's top leadership have been indicted in the U.S. Meanwhile, the FARC held onto a Cold War view of Colombia's political and economic establishment as "oligarchs" at the service of the U.S.
The rebel army was forced to the negotiating table after a decade of heavy battlefield losses that saw a succession of top rebel commanders killed by the U.S.-backed military and the its ranks thinned by half to the current 7,000 troops.
Santos, an unlikely peacemaker given his role as architect of the military offensive, throughout maintained a steady pulse even as he was labeled a traitor by his conservative former allies and suffered a plunge in approval ratings.
The most contentious breakthrough came in September when Santos traveled to Havana to lay out a framework for investigating atrocities, punishing guerrillas for involvement in those abuses and offering compensation to victims.
Opponents of Santos and some human rights groups harshly criticized a key part of that deal: guerrillas who confess their crimes won't spend any time in prison and will instead be allowed to serve out reduced sentences of no more than eight years helping rebuild communities hit by the conflict.
Another toad to swallow, as Santos calls the concessions he's had to make, will be the sight of former rebel leaders occupying seats in congress specially reserved for the FARC's still unnamed political movement. The exact number of such seats was among the last details being hammered out in marathon 18-hour sessions taking place in recent days.
"We haven't slept but it was worth the effort," said Sen. Roy Barreras, among political reinforcements sent in by Santos to work on the deal, speaking to Caracol Radio from Havana.
The announcement that talks have successfully concluded will trigger a series of events, some entailing political risks.
First, Santos must present the accords to congress and ask it to set a date for a plebiscite that could take place as early as next month. Polls show Colombians would likely endorse any deal in a simple yes or no vote.
But the still-unknown final accord may contain surprises and the opposition is likely to try to convert the vote into a referendum on Santos, whose approval rating plummeted to 21 percent in May according to a Gallup poll, the lowest since he took office in 2010.
Low voter turnout is also a concern because a minimum of 13 percent of the electorate, or about 4.4 million voters, must vote in favor for the accord to be ratified.
After the agreement is signed, the FARC will begin mobilizing its troops to 31 zones scattered across the country, and 90 days later they are supposed to begin handing their weapons over to United Nations-sponsored monitors.
But don't expect any immediate peace dividend or security improvements in Colombia's blood-splattered countryside.
Over the last 13 months, since the FARC declared a unilateral cease-fire and the government reciprocated with a truce of its own in all but name, violence has fallen to the lowest level since the FARC was created 52 years ago by outlaw peasant groups joined by communist activists. Only four deaths attributed to the FARC have been reported during that period and in the last 68 days the group hasn't carried out a single offensive action, according to a report last week by the Bogota-based Conflict Analysis Resource Center.
Analysts are concerned that as the rebels integrate into Colombian society, well-organized criminal gangs will fill the void and fight among themselves for control of the lucrative cocaine trade that kept the FARC well-armed much longer than other Latin American insurgencies.
While Colombia's homicide rate has fallen sharply over the years, it remains among the world's deadliest countries, with violence driven largely by its status as the world's top supplier of cocaine.
The much-smaller National Liberation Army will also remain active, although it's pursuing a peace deal of its own.