Salvadoran Armed Forces and pro-government paramilitaries launched an offensive to disrupt the activities of the Farabundo Martí National Liberation Front (FMLN).

Following the 1969 Football War between El Salvador and Honduras, the Organization of American States (OAS) negotiated a ceasefire that established an OAS-monitored demilitarized zone (DMZ) three kilometers wide on each side of the border.

When the Salvadoran Civil War began, many villages, including the hamlet Las Aradas, were abandoned and camps were formed within the DMZ on the Honduran side of the border to avoid harassment from the military, as well as the National Guard and paramilitary Organización Democrática Nacionalista (ORDEN), which did not cross the border.

[2] In early 1980, FMLN guerrillas organized several small Salvadoran border villages and provided rudimentary military training.

Following their return, twice National Guard and ORDEN troops advanced on Las Aradas, and twice the refugees fled across the river.

[4] They gathered and killed many refugees,[1] shooting them with machine guns,[1][4] bludgeoning them with rifle butts[4] or goring them with machetes and military knives.

[2] A few days after the massacre, the newspaper Tiempo published an interview with Father Roberto Yalaga, a priest in the diocese of Santa Rosa de Copán, who confirmed that at least 325 Salvadorians had been killed and that a Honduran military detachment had cordoned off the bank of the Sumpul river.

[8] On June 19, the diocese of Santa Rosa de Copán filed a formal complaint, signed by its 38 pastoral workers.

[2] Salvadoran Defense Minister José Guillermo García denied the massacre, stating, "There have been dead in that area, but not in such 'industrial' quantities.

[2] In October 1980, President José Napoleón Duarte, in an interview with United Church Observer, acknowledged that a military operation had taken place in the Sumpul river area and said that some 300 people, all of them "communist guerrillas", had died.

[2] On October 26, 1992, survivors of the Sumpul river massacre filed a judicial complaint with the Chalatenango Court of First Instance, which was admitted under the title "on verifying the murder of 600 people".

[1] In July 2016, when the Salvadoran Supreme Court struck down an amnesty law protecting participants in the civil war, enabling their prosecution, the case regarding the massacre remained open.