Edwin had ruled Northumbria since 616. Bede the Venerable, writing a century later, treats him as something close to a high king of Britain. He had brought Roman Christianity to the north when he was baptised at York in 627, married Aethelburh of Kent for that conversion, and extended his power across what Bede called the Mevanian Islands - Anglesey among them. Cadwallon, defending his Gwynedd kingdom, had been besieged on the island of Priestholm just off Anglesey's coast. He had then escaped, regrouped, and reversed the situation entirely, driving the Northumbrians out of Welsh territory. Then he allied with Penda of Mercia. The pagan Mercian and the Christian Welshman made strange partners against a Christian Northumbrian king, but politics in seventh-century Britain rarely respected the new religion. Cadwallon was the stronger partner. Bede's account suggests Penda was not yet king of Mercia at the time, though the Anglo-Saxon Chronicle dates his accession to 626.

What happened on the field itself can only be sketched. Seventh-century battles leave thin records, and Bede tells us little of the tactics. The location was marshy ground near the south bank of the Don - the kind of terrain that constrains cavalry and rewards defenders who know it. The Welsh and Mercians appear to have known it well. Edwin and Osfrith both fell in the fighting. Edwin's second son, Eadfrith, was taken alive by Penda. The captive prince was murdered later, despite a pledge of safe conduct. Edwin's head, in due course, was carried back to York and buried in the porch of the church he had begun building there - the first stone York Minster. Cadwallon then turned his army loose on Northumbria and spent the following year laying it waste, killing both of Edwin's successors and pursuing a campaign that Bede describes as genocidal in intent. He was finally stopped in 634 by Oswald, son of the exiled Northumbrian king Aethelfrith, at the Battle of Heavenfield near Hadrian's Wall.

The political consequences were almost as severe as the human ones. Northumbria had been a single kingdom under Edwin; after Hatfield it split back into its constituent halves of Bernicia in the north and Deira in the south. Eanfrith, a son of the dispossessed Aethelfrith line, returned from exile in Pictland to take Bernicia, while Edwin's cousin Osric took Deira. Both reverted from Christianity to paganism. Both were dead within a year, killed by Cadwallon's continuing campaign. It took the arrival of Oswald, another exile and a more committed Christian, to reunite the kingdom and stop the Welsh advance. The historian D. P. Kirby suggested that Hatfield was the outcome of a wide coalition opposed to Edwin's dominance, but if so the coalition did not survive the victory - Cadwallon and Aethelfrith's surviving sons turned on each other almost immediately.

The traditional location is the area around Hatfield, the village northeast of Doncaster, on what was once the great marshland called Hatfield Chase before its drainage by Cornelius Vermuyden in the 1620s. But the precise battlefield has never been securely identified, and one investigation group has challenged the location entirely. The Battle of Hatfield Investigation Society points instead to Cuckney in Nottinghamshire, where a place called locally High Hatfield - recorded historically as Cukeney upon Hattfeild - sits beside St Mary's Church. In the 1950s, workmen underpinning that church against coal-mine subsidence uncovered a mass grave that predates the twelfth-century building. The society received Heritage Lottery Fund grants in 2016 and 2018 for ground-penetrating radar and LIDAR surveys, but the Diocese of Southwell has refused permission for excavations. The bones remain undisturbed. Where Edwin fell remains, in the strictest sense, unknown.