The Battle of Tamarón took place on 4 September 1037 between Ferdinand, Count of Castile, and Vermudo III, King of León. Ferdinand, who had married Bermudo's sister Sancha, defeated and killed his brother-in-law near Tamarón, Spain, after a brief war. As a result, Ferdinand succeeded Bermudo on the throne.
Ferdinand's father, Sancho the Great, who was jure uxoris Count of Castile, had entrusted his son with the county as early as 1029. In 1034 he had conquered the imperial city of León and forced Bermudo to take refuge in remote Galicia. After Sancho died in 1035 Bermudo returned to León and launched a war for control of the Tierra de Campos, the territory between the Cea and Pisuerga rivers, long disputed with Castile (which was nominally a Leonese vassal). The dispute dates back to the tenth century. Under Sancho the Great the region had been united to Castile, and Ferdinand considered it as his wife's dowry. Recently, Gonzalo Martínez Díez has disputed this thesis, first enunciated by the Historia silense in the 1110s.
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