War of the Three Sanchos The War of the Three Sanchos ( ) was a brief military conflict between three Spanish kingdoms in 1065 – 1067 . The kingdoms were all ruled by kings who were first cousins : Sancho II the Strong , King of Castile ; Sancho IV Garcés , King of Navarre ; and Sancho Ramírez , King of Aragon , all grandsons of Sancho the Great . The primary source for the war is the thirteenth - century " Primera crónica general " . The brief war was ignited in part by the strife left over from the division of the kingdom of Sancho the Great in 1035 . That division had left Navarre with a supremacy over the " petty kingdoms " ( " regula " ) of Castile and Aragon , but by 1065 Navarre was a vassal of Castile ( now joined with the Kingdom of León ) . In 1065 Ferdinand the Great , the Castilian monarch died and his kingdom was divided between his sons , with the eldest , Sancho , taking Castile . Sancho of Castile was covetous of the lands of Bureba and Alta Rioja . Ferdinand had helped reconquer them from the Caliphate , but then had ceded them to his elder brother García Sánchez III of Navarre , the father of Sancho IV . After an initial series of frontier raids , Sancho IV of Navarre asked for an alliance from Sancho Ramírez of Aragon . Most of the war took place in the region of Burgos and La Rioja . The war was also fought over Castile ' s ability to take part in the Reconquista , a capacity which had been diminished by the division of Ferdinand ' s kingdom in 1065 . Sancho of Castile did try to extend his influence over the Muslim " taifa " of Zaragoza , which owed him " parias " . According to the twelfth - century " Crónica Najerense " , a battle was fought during which campaign his " alférez " , Rodrigo Díaz de Vivar , defeated his Navarrese counterpart , Jimeno Garcés , and gained the nickname " campi doctor " or " master of the field [ of battle ] " , later to become famous in Spanish literature as " el Campeador " . Between August and September 1067 Sancho Ramírez led a counterattack against Castile . Tradition is divided over who had the victory , the " Chronicle of San Juan de la Peña " attributes a rout to the Navarrese and Aragonese at Viana , while the " Primera crónica " attributes victory to Sancho of Castile . Ramón Menéndez Pidal and Bernard F . Reilly accept the latter tradition , Reilly citing a donation of December 1167 to the monastery of Oña by a " Flaino Oriolez dominator Tetelie " , a landholder in the Trespaderna district of the upper Ebro Valley . The participation of a Castilian magnate from the Navarrese border in an act by which Sancho formally defined the jurisdiction of the Diocese of Oca ( the only bishopric in Castile ) strongly suggests that Sancho was in a strong position . The chronicler of San Juan de la Peña , a Navarrese source , wrote that Sancho of Castile was forced to raise the siege of Viana and flee on a horse bedecked only in its halter ; that he subsequently convinced Abd ar - Rahman of Huesca to go to war with Aragon ; and that Sancho Ramírez eventually made peace with him anyway . Castile retook Álava , the Montes de Oca , and Pancorvo , as well as Bureba and Alta Rioja , but the conflict ended in a stalemate 1067 when the death of Sancha of León , Ferdinand ' s widow , opened the way to war between Ferdinand ' s sons . The central issue in the conflict , the possession of the border territories , was resolved in 1076 when Sancho IV was assassinated by his own brother and his kingdom partitioned between Sancho Ramírez , who became king of Navarre , and Alfonso VI of León and Castile , who received the disputed lands .