The western colony was beginning to raise itself into a great power, under its fierce and strong old king Penda, who seems to have consolidated all the petty chieftainships of the Midlands into a single fairly coherent kingdom.
Penda hated Northumbria, which, under Eadwine, had made itself the chief English state: and he also hated Christianity, which he knew only as a religion fit for Welsh slaves, not for English warriors.

For twenty-two years, therefore, the old heathen king waged an untiring war against Christian Northumbria.

In 633, he allied himself with Cadwalla, the Christian Welsh king of Gwynedd, or North Wales, in a war against Eadwine; an alliance which supplies one more proof that the gulf between Welsh and English was not so wide as it is sometimes represented to be.
The Welsh and Mercian host met the Northumbrians at Heathfield (perhaps Hatfield Chase) and utterly destroyed them.

Eadwine himself and his son Osfrith were slain.

Penda and Cadwalla "fared thence, and undid all Northumbria." The country was once more divided into Deira and Bernicia, and two heathen rulers succeeded to the northern kingdom.