The flag of the FARC (Revolutionary Armed Forces of Colombia) hangs in El Diamante, Colombia.
The decision to return to arms angered many Colombians, many of whom believe the FARC benefited from a sweetheart deal.
But a group of dissident FARC commanders never demobilized and have seen their ranks swell to around 2,500 fighters through recruitment.
In addition, the more radical National Liberation Army, or ELN, has filled the void left by withdrawing FARC rebels and stepped up attacks in cities, including the car bombing of a police academy in Bogota that killed 22 people.
Marin's move is "better late than never," said an ELN commander in Colombia's western jungle who goes by the alias Uriel.