The leftist rebels were done in by economics. When groups like FARC and ELN were founded nearly half a century ago, they had a lot of public support, and that meant the leftist gunmen could depend on voluntary contributions of food, clothing and other supplies. But over the decades, two things happened. First, popular support waned as decades went by and the leftist revolutionaries had little success. Then came the lure of the cocaine trade, which arose in the 1970s, as the traditional marijuana dealers began growing coca plants and manufacturing cocaine as well. Growing prosperity in North America and Europe created a large market for this more expensive, and easier to smuggle, drug. The drug gangs had money and needed protection from the police and army. The leftist rebels needed money, and already has a military organization out in the countryside. That cash enabled FARC to pay its troops, and overcome the falling voluntary support. FARC then began to use more extortion and other criminal methods to meet the growing payroll.
In the last decade, the government developed new tactics that attacked the logistics (supply lines) of the leftist rebels. By patrolling the routes used to move food and other goods to the rebel camps deep in the mountains and forests, the rebels went hungry, and without many other goodies. Months, or years, later, the army assault battalions would come after the rebels themselves, and often found their targets much weakened by desertion. In the last few years, an amnesty program has played lured many more fighters. These days, the special operations against the rural rebel camps often find few fighters left, and an organization that is wasting away.
As the rebels became weaker, they also found themselves attacked by the people they used to live off. By the 1980s, the locals began to form militias to defend themselves from the extortion and kidnapping activities of the rebels. The government disbanded the largest of these self-defense forces, the AUC, over the past few years, because the cure had become as bad as the disease. The AUC had gotten into the drug business, just like FARC had. But the AUC had roots in the community and could justify disbanding when the government restored order in the countryside. FARC is still a big organization (with about 6,000 armed, or at least full time, members.) But loss of public support and better government tactics have put the rebels on the defensive. They are now popularly considered just another criminal organization that, a long time ago, had some political purpose.
Some surviving FARC units (called "Fronts" and containing several hundred fighters at full strength) fled to more rural areas, especially along the borders (where one could flee to another country if the police and army got too close.) This has brought the leftists into conflict with people who had fled there before them. Colombia has over fifty major Indian tribes, who have survived centuries of outsiders and cultural change by fleeing to the fringes of society. Now desperate rebels are sharing the same space with desperate Indians, and the more heavily armed rebels tend to get their way at the expense of the tribes.
February 1, 2009: FARC released four of its "political" kidnap victims (a soldier, and three policemen). This was done with much drama and stage presence. FARC is trying to counter the revelations that it has, over the last few years, simply murdered hundreds of hostages when it discovered that their families were not able to raise the ransom demanded. FARC is also trying to use the hostage releases to hamper military operations (to demand a halt to patrols and raids in a large area so the hostages to be released can be moved.) The government is not giving in to this, and knows that FARC is eager to release hostages that are increasingly getting rescued by the army, or released by their guards (in return for amnesty and rewards.)
January 28, 2009: In the capital, a bomb went off in a Blockbuster video store, killing two people. The culprit was FARC, which is trying to maintain itself in urban areas via criminal activities (like extortion, and violence against those who don't pay). The government used this incident as an opportunity to publicize a new police unit that would concentrate on going after extortion operations (FARC or otherwise.)