Beltrán de la Cueva is, however, best known for allegedly having an affair with Henry's second wife, Joan of Portugal. It was rumoured that Henry's only child, Joan was fathered by Beltrán and not by the King himself, who may have been impotent. This rumour led to a four-year War of the Castilian Succession, which was won by Isabella I, Henry's half-sister. It is unlikely that an agreement as to Joan's probable paternity will ever be reached by historians, as there is not enough evidence to support either possible father with certainty. Most of the extant contemporary sources about Henry's potency are suspect, as the royal chronicles of his reign were either written or revised under the influence of Isabella, whose personal interest in the succession caused her to take great pains to insist on Joan's illegitimacy. Much of Isabella's attention to Henry, in fact, was spent on harming his reputation in order to cement the legitimacy of her own reign. The question of Joan's paternity has, as a result, fascinated historians for centuries: if Joan was not in fact Beltran's daughter, and was actually legitimate, Isabella's tremendously influential reign would have been an illegal usurpation.