Sancho III of Navarre and the Seizure of Castile (1016–1027 CE)

In 1016, Sancho III of Navarre defines the border between Navarre and Castile, strengthening his alliance with Count Sancho García of Castile through marriage to his daughter, Muña Mayor Sánchez (Muniadona).

Following Sancho García’s death, in 1017, Sancho III assumes the protectorship of Castile on behalf of young García Sánchez, further consolidating his influence over the region.

The Assassination of Count García Sánchez (1027 CE) and Its Aftermath

Relations between the Christian kingdoms of León, Castile, and Navarre deteriorate after the assassination of García Sánchez in 1027.

• García had been betrothed to Sancha, daughter of Alfonso V of León.
• As part of the marital pact, León was set to gain Castilian lands between the Cea and Pisuerga rivers, expanding its influence.
• However, upon arriving in León for the wedding, García Sánchez is murdered by the sons of a noble he had previously expelled from his lands.

Sancho III, who had opposed the marriage and the Leónese territorial expansion into Castile, seizes the opportunity to reverse the expected outcome.

Sancho III Takes Control of Castile

Using his role as protector of Castile as a pretext, Sancho III immediately occupies the county, asserting his authority over the region. He installs his younger son, Ferdinand, as Count of Castile, thus fully integrating the county into his sphere of influence and blocking León’s expansion.

This move significantly enhances Navarrese dominance in northern Iberia, setting the stage for the future rise of Castile as an independent power under Ferdinand I, who would later become King of León and Castile.