After publicly quarrelling with Charles de la Cerda in Paris at Christmas 1353, Charles arranged the assassination of the Constable, which took place at the village of l'Aigle (8 January 1354), his brother Philip, Count of Longueville leading the murderers. Charles made no secret of his role in the murder, and within a few days was intriguing with the English for military support against his father-in-law King John II, whose favourite the Constable had been. John II was preparing to attack his son-in-law's territories, but Charles's overtures of alliance to King Edward III of England led John instead to make peace with the King of Navarre by the Treaty of Mantes of 22 February 1354, by which Charles enlarged his possessions and was outwardly reconciled with John II. The English, who had been preparing to invade France for a joint campaign with Charles against the French, felt they had been double-crossed: not for the last time, Charles had used the threat of an English alliance to wrest concessions out of the French king.