The three groups go by initials, natch: FARC, M19, and ELN. The biggest, by far, is the FARC, with around 18,000 combat troops. That may not seem like a lot by US or Russian Army standards, but those are big conventional armies. Most "guerrilla" groups are real small. They have to be. They need to be mobile, keep their logistics simple, and be able to disappear fast. For that kind of fighting, you don't really need too many combat troops.
There are guerrilla groups with only a couple of dozen troops that work damn well. In fact, one of the weirdest war stories I ever heard was about a guerrilla army consisting of three guys. They were survivors of a Japanese platoon stranded in the Philippines, and their leader, this hard-ass sergeant, refused to surrender. He decided that it was his duty to the Emperor to kill anyone who entered his territory. For 30 years these three guys controlled a huge chunk of jungle, killing any of the locals who entered their domain. The Philippine Army couldn't find them; the villagers got the message and left them alone. In other words, they were an effective military force -- with three men.
So 18,000 men makes the FARC a huge army by guerrilla standards. They're by far the most aggressive of the three groups. Like most Latin American guerrilla armies, they have an elite command group who are almost all middle- or upper-class boys'n'girls, commanding troops who are almost all campesinos. (That's the way Colombia's army looks too: rich elite command, poor peasant troopers. It's got the usual Latin American pattern: 400 families own half the wealth, and everybody else lives on beans and rice. )
The two other rebel armies, ELN and M19 (the oldest group) haven't been run as well as FARC; they're seen as less powerful and clever and are just holding on. But the FARC really expects and intends to defeat the Colombian Army. This is pretty remarkable, because it's been a long time since a Maoist peasant army won anything. The Khmer Rouge were the last to do it, and that was 27 years ago. In 1950 the world was jammed with groups like this; now it's only in weird, fucked-up corners like Nepal and Colombia that they count for anything.
Al Pacino as Scarface: Colombian war for dummies.
The FARC's last big push was supposed to take the big cities -- the old Maoist battle-plan, where you strangle the cities and then march in. It failed. FARC lost a lot of soldiers and pulled back to its backwoods powerbases. The only city in Colombia where FARC has any control is Medellin. The slums of Medellin are FARC territory. But most of the troops are dispersed again, over the big tracts of jungle and scrub FARC run.
When the urban offensive failed, FARC decided to go for the soft target: the paramilitaries. The Army was trying to run a kind of Vietnamization program, get the paramilitaries to do some actual fighting -- and it worked about as well as it did in Vietnam: as in not at all. The FARC trapped big paramilitary forces in several towns and wiped them out. And I gotta say, it couldn't've happened to a nicer bunch of cowards, murderers and rapists. You can respect soldiers; you can respect guerrillas; but these assholes who dodge combat and slit peasants' throats for a living need to die, the sooner the better.
And that's how most Colombians felt too, because when the FARC switched tactics, hitting the paramilitaries instead of the Army, their popularity zoomed. So at the moment they're riding high, getting more peasant support every time they wipe out another bunch of these pigs.
The FARC is also one of the richest guerrilla groups this side of Al Fatah -- thanks to its own coke-'n'heroin growing and shipping business in the jungle areas it controls, and the Bush press whores like to claim that the rebels are responsible for all the coke killing our kids, blah blah blah. Bullshit. Every faction in the war grows, processes, ships and sells coke: the army, the "paramilitaries," the rebels, and a whole lot of poor scared peasants who just want a little money.