Colombia's Revolutionary Armed Forces (FARC) consist of 16,000 well-armed troops who control 40 per cent of national territory, most it sparsely populated.
In rural areas rebel leaders veto the appointment of government officials and negotiate border issues with Venezuelan customs, tacit acknowledgment of their status as sole authority in the area.
The roots of the FARC lie in armed self-defence groups which protected rural communities displaced by government violence. Colombia's liberal and conservative parties engaged in a bloody power struggle in 194852, which cost 250,000 lives.
Thousands of Colombians fled into remote hillsides where they formed self-sufficient "independent republics" controlled by Colombia's Communist Party, which refused to be drawn into the war.
The republics offered a peaceful if frugal farming existence to their members, who adapted to the tough mountain conditions, trading goods with communities living under government rule.
The two main parties eventually established a power-sharing arrangement in 1958 and set about destroying their communist rivals, fearing the threat of autonomous economic development beyond state control.
The government sent a third of the armed forces into the enclaves, which were effectively destroyed in 1963. A group of outraged farmers resisted the Colombian army and then turned professional in 1964, calling their group the FARC.
Unlike other rebel movements in Latin America, the FARC leadership is largely peasant-based, making it more difficult to buy off. In the past 37 years the FARC has undergone several major transformations, including one ill-fated attempt to launch a political party, Union Patriotica (UP).
This initiative in the 1990s failed when the entire active UP membership was picked off one by one by assassins as they attempted to engage in democratic politics.
The short-lived experiment with democratic politics gave way to a new militarist mood which coincided with the increase in funds provided by mass kidnappings and taxes levied on drug shipments in areas under guerrilla influence.
Colombia's cocaine trade has fuelled wars and corrupted democratic institutions as business people, politicians, judges and journalists are bought off or killed by drug lords.
The successful reinvention of the FARC, achieved mainly through military action, has produced a new generation of leaders who are largely indifferent to public opinion. The one-off peace process which began in January 1999 has so far achieved only a prisoner swap between rebels and police.
The FARC has pioneered a range of home-made explosives, including the bicycle-bomb, the donkey-bomb and the housebomb, the latter consisting of piles of explosives stored in a building close to a military target then detonated to coincide with a ground attack.
However, the media portrayal of the FARC as a gang of bloodthirsty, drug-dealing outlaws could just as well apply to the security forces sent to combat them.
The Colombian army has been indicted by international rights groups for "grave and persistent" human rights abuses, including torture and killing of political suspects. Right-wing paramilitaries, with logistical help from the army, have massacred entire villages, declaring anyone living in areas of rebel influence to be legitimate targets.
The FARC's charismatic leader, Manuel Marulanda, is an old man who has spent 50 years in the hills, outlasting a dozen presidents who swore to kill him.
The rebels have forced drug-traffickers to pay better wages to coca farmers and also intervened in labour disputes, burning Smurfit-owned vehicles in a dispute with the Irish multinational, which has offices in south-west Colombia.
The roots of the FARC may officially lie in the 1950s displacement of rural farmers, but the movement thrives today because inequality, poverty and state terror also flourish.