What gave life to these attacks and conspiracies was the hostility of France. The influence of the Duke of Burgundy was still strong enough to prevent any formal hostilities, but the war party was gaining more and more the ascendant. Its head, the Duke of Orleans, had fanned the growing flame by sending a formal defiance to Henry the Fourth as the murderer of Richard. French knights were among the prisoners whom the Percies took at Homildon Hill; and it may have been through their intervention that the Percies themselves were now brought into correspondence with the court of France. No house had played a greater part in the overthrow of Richard, or had been more richly rewarded by the new king. But old grudges existed between the house of Percy and the house of Lancaster. The Earl of Northumberland had been at bitter variance with John of Gaunt; and though a common dread of Richard's enmity had thrown the Percies and Henry together the new king and his powerful subjects were soon parted again. Henry had ground indeed for distrust. The death of Richard left the young Mortimer, Earl of March, next claimant in blood of the crown, and the king had shown his sense of this danger by imprisoning the earl and his sisters in the Tower. But this imprisonment made their uncle, Sir Edmund Mortimer, the representative of their house; and Edmund withdrew to the Welsh Marches, refusing to own Henry for king. The danger was averted by the luck which threw Sir Edmund as a captive into the hands of Owen Glyndwr in the battle of Brynglas. It was natural that Henry should refuse to allow Mortimer's kinsmen to ransom so formidable an enemy; but among these kinsmen Henry Percy ranked himself through his marriage with Sir Edmund's sister, and the refusal served as a pretext for a final breach with the king. [Sidenote: Overthrow of the Percies]