In 1377 he proposed to the English that he would return to Normandy and put the harbours and castles he still controlled there at their disposal for a joint attack on France; he also proposed that his daughter should be married to the new English King, the young Richard II. But the threat of an attack by Castile forced Charles to remain in Navarre. Instead he sent off his eldest son to Normandy, with a number of officials, including his chamberlain Jacques de Rue, who were to prepare his castles to receive the English, as well as a servant whose mission was to insinuate himself into the royal kitchens in Paris and poison the King of France. Meanwhile he urgently appealed for the English to send him reinforcements from Gascony to help him fight the Castilians. But in March 1378 all his plots finally unravelled. On their way to Normandy the Navarrese delegation were arrested at Nemours. The draft treaties and correspondence with the English found in their baggage, along with Jacques de Rue's confessions under interrogation, were all that Charles V needed to send an army into northern Normandy to capture all the King of Navarre's remaining domains there (April–June 1378). Only Cherbourg held out: Charles of Navarre begged the English to send him reinforcements there but instead they seized it for themselves and garrisoned it against the French. Charles's son submitted to the French King and became a protégé of the Duke of Burgundy, fighting in the French armies. Jacques de la Rue and other prominent Navarrese officials in France were executed.