===11th and 12th centuries: Expansion and union to the Kingdom of León===
In 1028, [[Sancho III of Navarre]] married the sister of Count García Sánchez and inherited the County of Castile after his brother-in-law's death. In 1035, Sancho III handed over the county to his son [[Ferdinand I of León|Fernando]] and at which time Castile acquired the status of an independent kingdom. Fernando I was married to Sancha, sister of [[Bermudo III of León]]. Fernando I, allying himself with [[Kingdom of Navarre|Navarre]], began a war with León. At the battle of Tamarón Bermudo III was killed, leaving no offspring. This allowed his brother-in-law, Fernando I to assume the kingship of León for himself, claiming his wife's right to the crown, and resulting in the first union of the kingdoms of León and Castile.
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