Founded in 1964 by Fabio Vásquez Castaño, the National Liberation Army (Ejército de Liberación Nacional--ELN) adopted a doctrine for insurrection inspired by the Cuban Revolution. During the mid-1960s, ELN activities centered on the department of Santander and included seizing temporary control of small towns, opening jails to free prisoners, robbing banks, and making antigovernment speeches in small villages throughout the country in an effort to gain recruits. The guerrilla organization gained international notoriety in 1966 when it recruited Father Camilo Torres, a well-educated Roman Catholic priest from a socially prominent family. Torres joined the ELN following his unsuccessful efforts at organizing a political opposition to the National Front government. Only four months after taking up arms, Torres was killed in a confrontation with an army patrol (see Trends Within the Church since the 1940s , ch. 2).

Although the ELN was considered the most effective of the country's guerrilla organizations, in the early 1970s it was decimated by the military's counterinsurgency campaign. By 1973 the armed forces claimed that they had "virtually destroyed" the ELN. Although the military severed the ELN's ties to its urban support network, the guerrillas had recouped their strength by mid-decade. In 1975 and 1976, the ELN engaged in several kidnappings, bank robberies, and assassinations, including the killing of Inspector-General of the Army General José Ramón Rincón Quioñes.

The ELN was the only major guerrilla organization that did not sign the 1984 cease-fire agreement. This refusal, along with the ELN's kidnapping of President Betancur's brother in an attempt to sabotage the peace talks, reportedly earned the organization a rebuke from Cuban leader Fidel Castro Ruz. Possibly as a result of Castro's stance of support for the peace talks, three ELN fronts reached a temporary cease-fire agreement with the government.

In the late 1980s, the ELN's size was estimated at 500. Its theater of operations included vast stretches of Colombia's eastern plains and portions of the departments of Norte de Santander, Santander, Bolívar, Cauca, and Antioquia, and the intendancy of Arauca. The ELN's activities included kidnappings and attacks on petroleum installations, pipelines, and exploratory drilling sites. Such attacks were carried out not only to disrupt the national economy but also to draw attention to the exploitation of Colombia's natural resources by foreign companies.

Data as of December 1988