Following the overthrow of the Monarchy in 1931, José Castillo was appointed to the newly raised Assault Guards, a para-military force intended to maintain security in urban areas and provide a counterweight to the long established and conservative Guardia Civil. Officers of the Asaltos were selected for their perceived loyalty to the new Republic. Castillo had partaken in the failed rebellion of October 1934.Ruiz, Julius. The'red Terror'and the Spanish Civil War. Cambridge University Press, 2014, p.39
