During his lifetime, he was the most important Christian monarch of the Iberian Peninsula, bearing, in various media, the title of rex Hispaniarum. Having gone further than any of his predecessors in uniting the divided kingdoms of Iberia, his life's work was undone when he divided his domains shortly before his death to provide for each of his sons. The Kingdom of Navarre existed for almost six centuries after his death, but was never as powerful again.
Sancho aspired to unify the Christian principalities in the face of the fragmentation Muslim Spain into the taifa kingdoms following the Battle of Calatañazor. In about 1010 he married Muniadona Mayor, daughter of Sancho García of Castile, and in 1015 he began a policy of expansion. He displaced Muslim control in the depopulated former county of Sobrarbe, and profited from the internal difficulties in Ribagorza to annex that county between 1016 and 1019, a conquest initiated before the 1017 death of his brother-in-law left his wife with a distant claim. In 1025 he received the submission, as vassal, of Raymond III of Pallars Jussà, who had also been a Ribagorza claimant. He also forced Berengar Raymond I of Barcelona to become his vassal, though he was already a vassal of the French king. Berengar met Sancho in Zaragoza and in Navarre many times to confer on a mutual policy against the counts of Toulouse.
Sancho III had opposed the wedding — and the ensuing Leonese expansion — and received a chance to act upon García's death. As the late count's brother-in-law, he immediately occupied Castile and was soon engaged in a full-scale war with León under Alfonso's successor, Vermudo III. The combined Castilian and Navarrese armies quickly overran Vermudo's kingdom, occupying Astorga. By March 1033, he was king from Zamora to the borders of Barcelona.
In 1034, even the city of León, the imperiale culmen (imperial capital, as Sancho saw it), fell, and there Sancho had himself crowned again. This was the height of Sancho's rule which now extended from the borders of Galicia in the west to the county of Barcelona in the east.
In 1035, he refounded the diocese of Palencia, which had been laid waste by the Moors. He gave the see and its several abbacies to Bernard, of French or Navarrese origin, to whom he also gave the secular lordship (as a feudum), which included many castles in the region.
Taking residence in Nájera instead of the traditional capital of Pamplona, as his realm grew larger, he considered himself a European monarch, establishing relations on the other side of the Pyrenees. He was assassinated at Aguilar de Bureba on 18 October 1035 and was buried in the monastery of San Salvador of Oña, an enclave in Burgos, under the inscription Sancius, gratia Dei, Hispaniarum rex.
Sancho VI of Gascony was a relative of Sancho of Navarre and he spent a portion of his life at the royal court in Pamplona. He also partook alongside Sancho the Great in the Reconquista. In 1010, the two Sanchos appeared together with Robert II of France and William V of Aquitaine, neither of whom was the Gascon duke's suzerain, at Saint-Jean d'Angély. After Sancho VI's death (1032), Sancho the Great extended his authority definitively into Gascony, where he began to mention his authority as extending as far as the Garonne in the documents issued by his chancery.
He introduced French feudal theories and ecclesiastic and intellectual currents into Iberia. Through his close ties with the count of Barcelona and the duke of Gascony and his friendship with the monastic reformer Abbot Oliva, Sancho established relations with several of the leading figures north of the Pyrenees, most notably Robert II of France, William V of Aquitaine, William II and Alduin II of Angoulême, and Odo II of Blois and Champagne. It was through this circle that the Cluniac reforms first probably influenced his thinking. In 1024 a Navarrese monk, Paterno from Cluny, returned to Navarre and was made abbot of San Juan de la Peña, where he instituted the Cluniac custom and founded thus the first Cluniac house in Iberia west of Catalonia, under the patronage of Sancho. The Mozarabic rite continued to be practiced at San Juan, and the view that Sancho spread the Cluniac usage to other houses in his kingdom has been discredited by Justo Pérez de Urbel. Sancho sowed the seeds of the Cluniac reform and of the adoption of the Roman rite, but he did not widely enact them.
Sancho also began the Navarrese series of currency by minting what the Encyclopaedia Britannica calls "deniers of Carolingian influence." The division of his realm upon his death, the concepts of vassalage and suzerainty, and the use of the phrase "by the grace of God" (Dei gratia) after his title were imported from France, with which he tried to maintain relations. For this he has been called the "first Europeaniser of Iberia.
His most obvious legacy, however, was the temporary union of all Christian Iberia. At least nominally, he ruled over León, the ancient capital of the kingdom won from the Moors in the eighth century, and Barcelona, the greatest of the Catalan cities. Though he divided the realm at his death, thus creating the enduring legacy of Castilian and Aragonese kingdoms, he left all his lands in the hands of one dynasty, the Jiménez, which kept the kingdoms allied by blood until the twelfth century. He made the Navarrese pocket kingdom strong, politically stable, and independent, preserving it for the remainder of the Middle Ages. It is for this that his seal has been appropriated by Basque nationalism. Though, by dividing the realm, he isolated the kingdom and inhibited its ability to gain land at the expense of the Moslems. Summed up, his reign defined the political geography of Iberia until its union under the Catholic Monarchs.
Despite this, the contemporary ecclesiastic Abbot Oliva only ever acknowledged Sancho as rex Ibericus or rex Navarrae Hispaniarum, while he called both Alfonso V and Vermudo III emperor. The first title considers Sancho as king of all Iberia, as does the second, though it stresses his kingship over Navarre alone as if it had been extended to authority over the whole Christian portion of the peninsula.
To the Moors, he was always only Baskunish, the "lord of the Basques."