Eadwine's conquests roused all his neighbours against him, and in their common fear of the Northumbrian sword, English and Welsh princes were for the first time found joining in alliance. Penda, King of Mercia, an obstinate heathen and a great foe of the gospel, leagued himself with Cadwallon, King of Gwynedd, the greatest of the Christian chiefs of Wales. Together they beset the realm of Eadwine, and the great King of Northumbria fell in battle with all his host, at Heathfield, near Doncaster (632). The Welsh and Mercians overran Northumbria after slaying its king, and Cadwallon took York and burnt it. The Northumbrians thought that Eadwine's God had been found wanting in the day of battle, and most of them relapsed into paganism in their despair. Paulinus, who had become the first Bishop of York, had to flee away into Kent, the only kingdom where Christians were safe for the moment. [Sidenote: =The Irish missionaries.--Columba.=] But ere very long the Northumbrians were saved from their despair. Eadwine and the ancient stock of the kings of Deira were swept away, but there were two princes alive of the royal house of Bernicia. Their names were Oswald and Oswiu, and during Eadwine's reign they had been living in exile. Their abode had been among those of the Scots who had crossed over from Ireland and settled on the coast of northern Britain, in the land which now bears their name. There the two brethren had fallen in with the disciples of the good Abbot Columba, the founder of the great monastery of Iona, and from them they had learnt the Christian faith. Columba, whose successors were to convert all the north of England, had been a man of great mark. He was an Irish monk who had left his own land in self-imposed exile, because he had been the cause of a tribal war among his countrymen. Crossing to the Argyleshire coast, he built a monastery on the lonely island of Iona, and from thence laboured for the conversion of the Picts and Scots. [Sidenote: =Oswald, King of Northumbria.--Christianity restored.=]