In the mid-1860s Sanz was assigned to batallón de Cazadores de Llerena; he served as teniente when in 1868 the unit was deployed against the revolutionaries in Béjar. The bloodbath and ensuing repression echoed across the country, which prompted Sanz and other officers to defend themselves in the press. Promoted to captain, he resumed teaching duties in Toledo. Sanz's career in the infantry college came to an abrupt end following declaration of the Republic; he lodged a discharge request and was released from the army. At that time he already declared himself a Carlist; straight from Toledo he headed North, where legitimist turmoil was gradually turning into a full-blown civil war. At that time his uncle, Cesáreo Sanz López, was already member of the highest Carlist military and administrative bodies.