In 1937, during the later stages of the Spanish Civil War, Bastico replaced Mario Roatta as the commander-in-chief of the Italian volunteer corps in Spain, the "Corpo Truppe Volontarie". The CTV was sent to help the Spanish Nationalists side in the war. From mid-1937, Bastico's force fought in the Battle of Santander, a decisive victory for the Nationalists. In late 1937, Bastico was replaced by Mario Berti. In October 1937, Bastico received the rank of general, "generale di corpo d'armata designato d'armata," the highest rank that could be assigned if Italy was not officially at war. In February 1939, the Italian volunteers left Spain.