In this article I examine Leonese nobility networks as a means to explain the involvement by Prince Alfonso of Castille (later King Alfonso X) in the civil war that confronted King Sancho II of Portugal with his brother Prince Alfonso, later Alfonso III of Portugal. Prosopography allows to reconstruct family bonds linking Leonese and Galician noblemen who had joined Prince Alfonso of Castille with their Portuguese counterparts who had rallied behind King Sancho. I will aim to prove that, following the alarm triggered by their Portuguese relatives, family bonds prompted Leonese and Galician noblemen to take part in the war and to seek Prince Alfonso of Castille’s support, after his father, King Ferdinand III, refused it.