As a condition for launching the talks, the FARC agreed to halt ransom kidnappings, the senior official said. It also agreed for the first time, the official said, to disarm at the signing of a peace agreement. Demobilized rebels would then take part in everything from destroying coca crops to launching political movements.
The talks will now proceed, outside Colombia, without any halt in combat and without any safe havens.
The Andean nation's internal conflict, which has claimed tens of thousands of lives, has not persisted a half-century by accident. It is maddeningly complex in a country with one of the world's widest gaps between rich and poor and the second largest internally displaced population after Sudan.
Potential spoilers to a peace deal abound, particularly to the agrarian reform and rural development that Santos, a social progressive, says would be part of a successful deal.
Resistance can be expected from wealthy ranchers and plantation owners allied with Alvaro Uribe, who as president in 2002-2010 waged war without quarter against the FARC while making peace with far-right militias that did most of the dirty war killing.
And then there is drug trafficking.
It fuels all of Colombia's illegal armed groups: the rightist militias, their successor gangs and the FARC itself.
Five of the six members of the FARC's ruling Secretariat, including Jimenez, are deemed major drug traffickers by the US State Department, which has $5 million bounties out for each of them.
It is not clear how Washington would deal with them if the conflict ends. Colombia's congress passed a law in June that sets a framework for amnesties and pardons for rebel leaders.
Police and soldiers in Colombia mixed up in the illegal drug trade aren't exempt from prosecution. Would FARC commanders get a waiver?
Pastrana, who served as Colombia's ambassador to Washington after his presidency, called drug trafficking "a very important element" of peace talks and said "it would be good to invite the United States as well into this process."
Washington is a close ally of Colombia, but Santos has already exhibited considerably more independence than Uribe.