At the beginning of the Spanish Civil War in 1936, at the age of 18, Sanz Bocos enlisted in the Republican army, in the Antigas unit, starting at the front of Villanueva del Pardillo, to later be assigned to the Talavera front. He was part of a group sent to the pilot school in Kirovabad, Azerbaijan, where he was chosen to become pilot of the Polikarpov I-16.