Sancho was the eldest son and heir of García Sánchez III and his wife Stephanie. García was killed at the Battle of Atapuerca on 1 September 1054 during a war with the Kingdom of León. Sancho, who was then fourteen years of age, was proclaimed king by the army in the camp by the field of battle with the consent of the king of León, Ferdinand I, also his uncle. Sancho's mother served as his regent until her death on 25 May 1058. Remaining faithful to her husband's policies, she continued to support the monastery of Santa María la Real of Nájera. Soon after Sancho's accession, many lords in the west of the kingdom went over to the Leonese. Only Íñigo López, lord of Biscay, and Sancho Fortúnez, lord of Pancorbo, remained loyal. On 29 December 1062, Sancho and Ferdinand signed a treaty defining their shared border. Ferdinand was recognised as king of all Castile and Sancho's authority was recognised in the Rioja, Álava, Biscay, and implicitly Guipúzcoa. As king, Sancho received support from his other uncle, King Ramiro I of Aragon. Out of gratitude for "his friendship, his fidelity, his help and his council", Sancho gave Ramiro possession of Lerda, Undués and the castle of Sangüesa. These places were probably to be held as fiefs or in a similar arrangement. Beginning in 1060, Sancho put pressure on al-Muqtadir, king of Zaragoza, and exacted from him annual payments of tribute, parias.
War and assassination
From 1065, he was in conflict with Castile, raised to a kingdom for Ferdinand's son Sancho the Strong. This culminated in the so-called War of the Three Sanchos. Years before, Sancho's father had managed to retain a series of frontier lands, including Bureba and Alta Rioja, which had been claimed by Ferdinand. Sancho the Strong sought to reconquer these lands for his kingdom. Faced with an invasion by his cousin the Castilian Sancho, The Navarrese Sancho asked for aid from his other cousin, Sancho of Aragón. But their forces were defeated by Sancho the Strong and his trusted alférezEl Cid, Sancho lost Bureba, Alta Rioja, and Álava to Sancho of Castile. He was assassinated in Peñalén, whence his nickname, by a conspiracy headed by his brother Ramón Garcés and his sister Ermesinda of Navarre. During a scheduled hunt, Sancho was forced from a cliff by his siblings. Upon his assassination, the kingdom was invaded and ultimately partitioned between Sancho of Aragon and a third cousin, Alfonso VI of León and Castile. Alfonso occupied La Rioja and Sancho was proclaimed king in Pamplona.
García Sánchez, with the same name as the eldest son, dead after 1092. His existence is confirmed on a diploma from the Monastery of Valvanera dated in 1092, which states Garsea et alter Garsea, germani, filii Sanchii regis Nagerensis.
Sancho Garcés had a lover named Jimena with whom he had two illegitimate children:
