When Oswald heard of the desperate condition of Northumbria after Eadwine's death, he resolved to go to the aid of his countrymen against the Welsh and Mercians. So he went southward with a few companions, and raised the Bernicians against their oppressors, setting up as his standard the cross that he had learnt to reverence in Iona. His effort was crowned with success, and at the Heavenfield, near the Roman wall, he completely defeated the Welsh and slew their king Cadwallon. Penda the Mercian was driven out of Northumbria also, and for eight years (634-642) Oswald maintained himself as king of all the land between Forth and Trent. He used his power most zealously for the propagation of Christianity. He sent to Iona for two pious monks, Aidan and Finan, who were successively bishops of York under him, and by their aid he so drew his people toward the faith of Christ that they never swerved from it again, as they had done after the death of Eadwine. Oswald also encouraged missionaries to go into the other English kingdoms. It was by his advice that Birinus went from Rome to Wessex, where he converted King Cynegils, and founded the bishopric of Dorchester-on-Thames. [Sidenote: =Penda, King of Mercia.--Battle of Maserfield, 642.=] But Oswald was not strong enough to put down his heathen neighbour, Penda, the King of Mercia, a mighty warrior who united all the English of central Britain under his sceptre, slaying the kings of the East Angles, and tearing away Gloucester and all the land of the Hwiccas[3] from the kings of Wessex. Penda and Oswald were constantly at war, and at last the Mercian slew the Northumbrian at the battle of Maserfield, in Shropshire, near Oswestry (642). [Sidenote: =Oswiu, King of Northumbria.--Conversion of Mercia.=] But the good King Oswald left a worthy successor in his brother Oswiu, as zealous a Christian and as vigorous a ruler as himself. Oswiu defeated Penda at the battle of the Winweed, and by slaying the slayer became the over-king of all England. He conquered the Picts between Forth and Tay, made the Welsh and the Cumbrians pay him tribute, and annexed northern Mercia, giving the rest of the kingdom over to Peada, Penda's son, only when he became a Christian. It was all over with the cause of heathenism when Penda fell, and the Mercians and their king bowed to the conquering faith, and listened to the preaching of Ceadda, one of the Northumbrian monks who had been taught by the Irish missionaries Aidan and Finan. [Sidenote: =Dissensions of Irish and Roman clergy.--Council at Whitby, 664.=]