Percy had withdrawn from the Welsh war in wrath at the inadequate support which Henry gave him; and his anger had been increased by a delay in repayment of the sums spent by his house in the contest with Scotland, as well as by the king's demand that he should surrender the Earl of Douglas whom he had taken prisoner at Homildon Hill. He now became the centre of a great conspiracy to place the Earl of March upon the throne. His father, the Earl of Northumberland, his uncle, Thomas Percy, the Earl of Worcester, joined in the plot. Sir Edmund Mortimer negotiated for aid from Owen Glyndwr; the Earl of Douglas threw in his fortunes with the confederates; and Henry Percy himself crossed to France and obtained promises of support. The war party had now gained the upper hand at the French court; in 1403 preparations were made to attack Calais, and a Breton fleet put to sea. At the news of its presence in the Channel Henry Percy and the Earl of Worcester at once rose in the north and struck across England to join Owen Glyndwr in Wales, while the Earl of Northumberland gathered a second army and advanced more slowly to their support. But Glyndwr was still busy with the siege of Caermarthen, and the king by a hasty march flung himself across the road of the Percies as they reached Shrewsbury. On the twenty-third of July a fierce fight ended in the defeat of the rebel force. Henry Percy was slain in battle, the Earl of Worcester taken and beheaded; while Northumberland, who had been delayed by an army under his rival in the north, Neville, Earl of Westmoreland, was thrown into prison, and only pardoned on his protestations of innocence. The quick, hard blow did its work. The young Earl of March betrayed the plans of his partizans to purchase pardon. The Breton fleet, which had defeated an English fleet in the Channel and made a descent upon Plymouth, withdrew to its harbours; and though the Duke of Burgundy was on the point of commencing the siege of Calais the plans of an attack on that town were no more heard of. [Sidenote: Henry's difficulties]