In response, peasants in those provinces formed self-defence groups, backed by conservative landowners, to protect themselves against the guerrillas, who engaged in kidnapping, sabotage and extortion. Fuelled by Colombia's drugs boom during the 1980s, these groups gradually evolved into the 10,000-strong right-wing paramilitary umbrella group, the United Self-Defence Groups of Colombia (AUC).
The paramilitaries, government troops and guerrillas battle for territorial control, particularly in the southern provinces and along Colombia's borders.
In August 2002, Alvaro Uribe was elected president with a large majority to crush the guerrillas. He stood as an independent liberal candidate but his critics consider the government to be right wing and increasingly authoritarian. Nevertheless, Mr Uribe remains a trusted leader with a 74 per cent approval rating. Many believe he is defeating the guerrillas and feel that road security has improved.
Colombia's main ally is the United States, which gives billions of dollars of aid to combat terrorism and drugs trafficking.
Last year, the government negotiated a peace process with the AUC that resulted in the demobilisation of thousands of paramilitaries.
Mr Uribe wants the constitution changed to allow him to stand for a second term. The historic bid will be decided later this year.