Battle of Tamarón

Ferdinand's father, Sancho the Great, who was jure uxoris Count of Castile, had entrusted his son with the county as early as 1029.

After Sancho's death in 1035 Vermudo 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).

Autopsies performed in the twentieth century showed that he had received forty lance wounds, many in the lower abdomen, typical for dismounted knights.

According to the Historia silense, Chronica naierensis, and Chronicon mundi, Vermudo "crossed the Cantabrian border" (transiecto Cantabriensium limite), i.e., the Pisuerga, and engaged Ferdinand super vallem Tamaron.

The earliest source to explicitly date the battle is Pelagius of Oviedo, who writes that Vermudo died in his tenth year (which he incorrectly makes 1032, though it was in fact 1037).