Utilising millions in US aid, Alvaro Uribe, Colombia's president, has reduced violence by sending troops to take areas under rebel control and demobilise right-wing militias who once fought guerrillas.
Despite government efforts, the Revolutionary Armed Forces of Colombia (FARC), the country's largest left-wing rebel group, is still strong in rural areas.
US and Colombian officials say FARC is a drug-trafficking terrorist organisation.
A car bomb destroyed most of a milk storage plant run by Nestle, a Swiss company, on Wednesday after FARC killed five officers in a bomb attack on a police truck near the border with Ecuador.
More than 600 troops and police were killed in combat last year and thousands of people are still forced from their homes each year by fighting between state forces and rebels.
Colombia is the world's top producer of cocaine, with most of its crop destined for the United States and Europe.