But Zumalacárregui, who was noted for his grave and silent disposition and his strong religious principles, disliked the disorderly life of the guerrillas, and when regular forces were organized in the north he entered the 1st battalion of Gipuzkoa as an officer. During the remainder of the war he served in the regular army. His brother Miguel Antonio de Zumalacárregui was in Cádiz at the time the Cortes passed the Constitution of 1812, going on to be elected chief deputy of Gipuzkoa. Tomas was then sent with dispatches to the Regency at Cádiz, and received his commission as captain. In that rank he was present at the Battle of San Marcial in August 1813. After the restoration of Ferdinand VII he continued in the army, and is said to have made a careful study of the theory of war.