As part of the Reagan administration's military assistance to train rapid response units, the Atlacatl Battalion was activated in March 1, 1981, under the command of the daring and ambitious Lieutenant Colonel Domingo Monterrosa Barrios. Monterrosa had been in the same tanda class at the Salvadoran military academy as Roberto D'Aubuisson and had taken courses at the U.S. Army's School of the Americas in 1966. The Atlacatl was the first specialty battalion trained for counterinsurgency warfare in the Salvadoran armed forces. Under the watch of U.S. Army Special Forces advisors in El Salvador, the battalion completed its training in early 1981. The battalion grew to around 1,000 soldiers and became notorious for its violence and lack of discipline. U.S. trainers were privately enthusiastic about its combat potential. In early December of this same year, the battalion participated in Operación Rescate (Operation Rescue), an attempt to corner and annihilate the FMLN training camp at a locale called La Guacamaya.

Almost a year before Rescate took place, a company of the Atlacatl Battalion, commanded by Captain Juan Ernesto Méndez, had initiated a counterinsurgency mission in the northern zone of Morazán – the base of operations of Joaquín Villalobos's formidable ERP wing of the now united FMLN. The unit came under heavy guerrilla attack and quickly retreated. This deeply embarrassing setback for the brand new “Rapid Deployment Infantry Battalion” elicited jokes by officers of other units who nicknamed it the “Rapid Retreat Infantry Battalion.” The ignominious retreat might help explain at least some of the fury that accompanied Atlacatl's Rescate operation in December.

Revenge Would Be Enacted

On December 9, 1981, skirmishes took place between FAES forces and FMLN guerrillas in the village of Arambala, deep in the Morazán Department. That same day, a company of the Atlacatl Battalion entered Arambala and rounded up the population in the town square, separating the men from the women and children. Accused of being rebel collaborators, several villagers were blindfolded, tortured, and killed. Atlacatl was smelling blood and its revenge against leftist insurgents would be enacted on the villages and villagers whom they believed were abetting the enemy.