Sancho IV
With the death of his father a period of conflict started between the territories of Biscay and Castile. Diego López joined the side of Navarre and Aragon supporting the pretender to the throne Alfonso de la Cerda in the larger conflict to fight against Sancho IV of Castile. The war went poorly for Diego's side as the forces of Sancho IV began taking large swaths of territory, including the towns of Labastida, Orduña-Urduña, and Balmaseda. After a protracted conflict, Sancho IV was able to occupy Biscay.