Kingdom of Castile
In 1028 Sancho III the Great, of Navarre, married the sister of count García Sánchez and inherited title to the County of Castile after his brother-in-law's death. In 1035 he left the county to his son Fernando. Fernando I was married to Sancha, sister of Bermudo III of León. Fernando I began a war with León and in the battle of Tamarón against a coalition of Castile and Navarre the king of León was killed, leaving no offspring. His brother-in-law Fernando took the crown of León for himself and their three sons took the kingdoms of León (Alfonso VI), Galicia (García) and Castile, Sancho, becoming king and borning the Kingdom of Castile.