When the Spanish Civil War commenced in 1936, he rejoined the army on 18 July under general Gonzalo Queipo de Llano, commanding groups of falangists operating in working-class suburbs of Seville with artillery fire. Most of the falangists were from the "", a Sevillian aristocratic landowners' organisation which he became president of in the 1940s. On 8 August, leading the 9th battery of the third light regiment, he joined the ruthless commander Antonio Castejón Espinosa. Alarcón de la Lastra directed artillery which bombarded towns as part of the Nationalists' advance; bombing from the air was also used. Where the resistance to the bombardment was strong it was followed up with the usage of human shields, but in any case the abuse and killings were extreme. Alarcón de la Lastra was thereafter involved in all actions on the march to Madrid, including the Battle of Badajoz and the capture of Toledo. This advance was later under the command of Juan Yagüe (known as the "Butcher of Badajoz" after that particular ensuing massacre), whose forces had joined the Nationalists from Ceuta. On 19 November, he was injured twice at the front of his battery. Thereafter he remained at the vanguard or in charge of artillery as in Teruel in the Galician army corps. In 1937, he was promoted first to commander and then lieutenant-colonel. By late 1937, he was in command of the Moroccan Army Corps artillery.