His action at once changed the disaffection into a national revolt. His raids on the Marches and his capture of Radnor marked its importance, and Henry marched against him in the summer of 1401. But Glyndwr's post at Corwen defied attack, and the pressure in the north forced the king to march away into Scotland. Henry Percy, who held the castles of North Wales as Constable, was left to suppress the rebellion, but Owen met Percy's arrival by the capture of Conway, and the king was forced to hurry fresh forces under his son Henry to the west. The boy was too young as yet to show the military and political ability which was to find its first field in these Welsh campaigns, and his presence did little to stay the growth of revolt. While Owen's lands were being harried Owen was stirring the people of Caermarthen into rebellion and pressing the siege of Abergavenny; nor could the presence of English troops save Shropshire from pillage. Everywhere the Welshmen rose for their "Prince"; the Bards declared his victories to have been foretold by Merlin; even the Welsh scholars at Oxford left the University in a body and joined his standard. The castles of Ruthin, Hawarden, and Flint fell into his hands, and with his capture of Conway gave him command of North Wales. The arrival of help from Scotland and the hope of help from France gave fresh vigour to Owen's action, and though Percy held his ground stubbornly on the coast and even recovered Conway he at last threw up his command in disgust. A fresh inroad of Henry on his return from Scotland again failed to bring Owen to battle, and the negotiations which he carried on during the following winter were a mere blind to cover preparations for a new attack. So strong had Glyndwr become in 1402 that in June he was able to face an English army in the open field at Brynglas and to defeat it with a loss of a thousand men. The king again marched to the border to revenge this blow. But the storms which met him as he entered the hills, storms which his archers ascribed to the magic powers of Owen, ruined his army, and he was forced to withdraw as of old. A raid over the northern border distracted the English forces. A Scottish army entered England with the impostor who bore Richard's name, and though it was utterly defeated by Henry Percy in September at Homildon Hill the respite had served Owen well. He sallied out from the inaccessible fastnesses in which he had held Henry at bay to win victories which were followed by the adhesion of all North Wales and of great part of South Wales to his cause. [Sidenote: The Percies]