Alfonso V died in 1028 while besieging the city of Viseo (Portugal). He had two sons: Sancha and Bermudo. Since during that time it was prohibited for women to rule, Bermudo was crowned king, becoming the third Asturleonese king with that name. He was only eleven years old, so his stepmother Urraca governed on his behalf. As was common when a minor became king, the nobility dedicated themselves to rebel and loot the kingdom. Sancho Garcés III, king of Pamplona, took advantage of this chaotic situation to take hold of the region of Castille and other Leonese territories, including the capital and the city of Astorga. Bermudo III recovered these territories in 1035, but before that he had to allow the marriage of his sister Sancha with Fernando, the fourth and last one of Sancho Garcés’ sons. The Pamplonese king passed away at the end of 1035, and left the region of Castille to Fernando, even though this new count was subject to the Leonese king.
The last of the Viking raids occurred during Bermudo’s reign. Although there are very few documents about this king, from them it can be deduced that he suppressed many rebellions and was able to control the nobility.
Reasons remain unknown, but in 1037 Bermudo and his brother-in-law count Fernando declared war against each other. Fernando’s brother, king of Pamplona, helped him and together they fought against Bermudo in Tamarón, near the city of Burgos, on August 30th. Bermudo was 20 years old, and rode his horse foolishly against enemy armies. His horse Pelayuelo, famous for his speed, reached Castilian and Navarran lines much faster than the rest of the Leonese army. Bermudo found himself surrounded by the enemy and was attacked and killed. His legitimate successor was his sister Sancha, but since she was married to Fernando, both received the Leonese crown.