Edward's claim--such as it was--rested on the assertion that his mother, Isabella, was nearer of kin to her brother Charles IV., the last king of the main line of the house of Capet, than was Charles's cousin Philip of Valois. The French lawyers ruled that heiresses could not succeed to the crown themselves, but Edward pleaded that they could nevertheless transmit their right to their sons. He found it convenient to forget that the elder brother of Charles IV., King Louis X., had left a daughter, whose son, the king of Navarre, had on this theory a title preferable to his own. This prince, he said, had not been born at the time of his grandfather's death, and so lost any rights that might have passed to him had he been alive at that time. A far more fatal bar to Edward's claim than the existence of Charles of Navarre was the fact that the peers of France, when summoned to decide the succession question nine years before, had decided that Philip of Valois had the sole valid claim to the crown, and that Edward had then done homage to him for Guienne. If he pleaded that in 1328 he had been the mere tool of his mother and Mortimer, he could be reminded of the unfortunate fact that in 1331, after he had crushed Mortimer, and taken the power into his own hands, he had deliberately renewed his oath to King Philip. Edward's claim to the French crown embittered the strife in a most unnecessary fashion. It was an appeal to every discontented French vassal to become a traitor under a plausible show of loyalty, and from first to last many such persons utilized it. It also gave Edward an excuse for treating every loyal Frenchman as guilty of treason, and, to his shame, he did not always refrain from employing such a discreditable device. Yet, as has been already said, he showed his consciousness of the fallacy of his claim by offering to barter it again and again during the course of the war for land or money. But he finally passed on the wretched fiction as a heritage of his descendants, to cause untold woes in the 15th century. It is seldom in the world's history that a hollow legal device such as this has had such long enduring and deplorable results. Battle of Sluys.