Having gained little or nothing from these activities, he returned to Navarre in early 1372. He was subsequently involved in at least two attempts to have Charles V poisoned and encouraged various plots by others against the French King. He next entered into negotiations with John of Gaunt, who was aiming to make himself King of Castile by virtue of his marriage to Pedro I's daughter Constanza. But in 1373 Henry of Trastámara, now firmly installed as King of Castile and victorious in war against England's ally Portugal, forced Charles of Navarre to agree to a marriage alliance, to surrender the disputed border fortresses he had held on to since the Castilian civil war, and to close his borders to any army of John of Gaunt. Nevertheless in March 1374 Charles met John of Gaunt in Dax in Gascony and agreed to let him use Navarre as a base for invading Castile on condition he recapture the towns surrendered to Henry. Gaunt's sudden decision only a few days later to abandon his plans and return to England Charles took as a personal betrayal. In order to placate the Castilian King he now agreed for his eldest son, the future Charles III of Navarre, to marry Henry of Trastámara's daughter Leonora in May 1375.