Oswald had spent the eight years before Maserfield extending Northumbrian power across most of Britain south of the Humber. He came to the throne after killing Cadwallon ap Cadfan of Gwynedd at Heavenfield in 634 - revenge for the death of his uncle Edwin at Hatfield Chase the year before. Penda of Mercia had been Cadwallon's ally at Hatfield, and Bede suggests Penda recognised Oswald's overlordship in some form afterwards. But the relationship was tense. Mercia, lodged between Northumbria and the Welsh kingdoms, was permanently squeezed by both. By 642 Oswald and Penda were enemies again, and the question was which would strike first.

Tradition places the battle at Oswestry in Shropshire. The town's English name - first attested in 1191 as Oswaldestroe - means 'Oswald's Tree.' The traditional Welsh name, Croesoswallt, means 'Oswald's Cross.' Both names suggest a memory of a slain Christian king, but the identification is not certain. Bede calls the place Maserfelth, and the etymology of the first element remains debated: it might be a unique attestation of the Old English word for a maple-wood drinking cup, or related to Welsh maes ('field'), or the Welsh female name Meiser. In 2020 Andrew Breeze argued for Forden in Powys instead, based on place-name evidence in the 9th-century Welsh poetic cycle Canu Heledd. If Oswestry is the right site, it was probably still in Powysian territory in the 7th century - meaning Oswald had marched deep into hostile country.

The simple picture - Christian Northumbrians against pagan Mercians - leaves out the Welsh. The Canu Heledd suggests that warriors from Pengwern (the British kingdom centred near modern Shrewsbury) fought on Penda's side, probably as allies. They would have been Christians fighting alongside a pagan against a fellow Christian. The Welsh name Cogwy for the battle, and the place-name Dyffryn Meiser in the poetic cycle, anchor the British memory of the fighting separately from the Anglo-Saxon record. The Historia Brittonum, written from a British perspective, credits Penda's victory to 'diabolical agency' - but it also makes clear that Welshmen were part of the winning side. Eowa, Penda's brother and possible rival, may have died in the battle too, removing an internal threat to Penda's authority over the Mercians.

What Penda did to Oswald's corpse was, by 7th-century standards, theological as much as military. Dismembering a king's body and mounting the head and arms on stakes was an act of public humiliation, and it was also - perhaps - a pagan offering to the war-god Woden. It backfired badly. Oswald's brother Oswiu retrieved the pieces a year later. The arm and hand were taken to Bamburgh and reportedly remained uncorrupted. Oswald was venerated almost immediately, then formally canonised. Within a century, bones and relics attributed to him were being distributed across England, into Germany, into Italy. Bede listed miracles attributed to Oswald's bones and to the very ground where he had fallen. A spring at Oswestry, called Oswald's Well, was said to have water with healing properties - particularly for eye trouble. Henry of Huntingdon, writing in the 12th century, recorded a chilling local tradition: 'It is said the plain of Maserfeld was white with the bones of the Saints.'

Maserfield broke Northumbria. The kingdom split: Deira in the south chose its own king, Oswine; Bernicia in the north went to Oswald's brother Oswiu. They quarrelled. Oswiu had Oswine murdered in 651. The fractured Northumbria did not reunite as a serious power until after the Battle of the Winwaed in 655 - the battle in which Penda himself was finally killed, by Oswiu, and Mercian dominance ended. Frank Stenton, the great historian of Anglo-Saxon England, called Penda after Maserfield 'the most formidable king in England.' For thirteen years he raided as far north as Bamburgh, the Northumbrian royal capital. Then it ended for him, too. The Anglo-Saxon political world after 642 was different in shape from the one before: the Mercian midlands, briefly, were the centre of gravity. The town that eventually grew on the field where Oswald fell never forgot whose tree it was named for.