Following the Francoist revolt and the start of the Spanish Civil War in 1936, Carballo Calero became a volunteer combatant for the Spanish Republic, achieving the rank of lieutenant in the Spanish Republican Army. He was captured by the Francoists and convicted on the grounds of "secessionism" and sentenced to sixteen years in the jail of Jaén (Spain). Yet, he was released in 1941. He returned to his natal city of Ferrol, where he worked as a private tutor as he was banned from holding any public position, including teaching.