Following the outbreak of the Civil War, he joined the Guatemalan Workers Party (Partido Guatemalteco de los Trabajadores or PGT) guerrilla group. During this time he was arrested, tried, and jailed, after which he spent seven years in exile in Mexico. He returned to Guatemala in 1971 and helped form the Revolutionary Organization of the People in Arms (Organización Revolucionaria del Pueblo en Armas, ORPA). He fought under the "nom de guerre" Gaspar Ilom, which he took from a character in "Hombres de maíz", one of his father's novels. When four guerrilla groups, including these two, combined to create the Guatemalan National Revolutionary Unity (Unidad Revolucionaria Nacional Guatemalteca, URNG) in 1982, Asturias emerged as one of the four leaders of its general command. He was the only one of the leaders not to participate in signing the peace agreement reached with the government in the early 1990s.