Northumbria was the only Anglo-Saxon kingdom in Britain to remain strong after the disastrous Battle of Penn Hill thirty years earlier, which resulted in the British reconquest of much of southern Albion. Its warbands were strengthened by the addition of Saxon refugees from the south, which it used to subjugate the northern British kingdoms of Rheged and Strathclyde and to assert its dominance in Pictland. By 696 therefore it was feeling quite secure.
Powys meanwhile had been expanding into former Mercian territory, until it stretched almost all the way to the North Sea. By the 690s it had begun assimilating the remaining Angles and Saxons into British culture, and was eager to challenge the last possible threat to its dominion over the heart of the isle.
King Gwylog of Powys and his ally Idwal of Gwynedd led a strong force of spearmen across the river Mǽresea and marched into the Pennines towards the eastern coast. Along the way, they were joined by a number of British-speaking Elmetians, whose kingdom had been subjugated by Northumbria some eighty years earlier. King Aldfrith of Northumbria gathered his warbands and prepared to fight. Deducing that the Britons would be low on supplies by the time they left the Pennines, he decided to fight a war of attrition and ordered that all the land in their path be stripped of food and the wells poisoned.
The Britons approached the Northumbrian capital of Eoforwic but did not have enough supplies to spend time besieging it. Instead, they sacked a number of outlying villages and began the retreat back to the west. Aldfrith, urged on by his son Osred and some of his counsellors, decided to abandon caution and marched to face the British army at the edge of the Forest of Loidis in eastern Elmet.
Osred led a group of Northumbrian warriors towards the centre of the Powysian lines, and broke their shield-wall. The fyrd, scenting victory, followed him.
However, Idwal's men of Gwynedd were able to form a shield-wall towards the rear to halt the rout, while the broken Powysian wings reformed on either side of the advancing Northumbrians. Throwing themselves back into the fight, they managed to surround the bulk of the Northumbrian army and shattered it, achieving a great victory.
King Aldfrith fled the battlefield, but was captured and brought before King Gwylog ap Beli. The latter pardoned him, but decided to keep him as a hostage for the remainder of his life.
Eadwulf of Bernicia and Osred Aldfrithson took command of the Northumbrian forces. The former however retreated north to Bernicia and took no further part in the campaigning, and the latter, after suffering another defeat, fled overseas to Frankland.
In August Powysian forces entered Eoforwic and installed Gruffydd ap Cadwaladr as a client King of Ebrauc. Ebrauc later achieved full independence from Powys and became one of the most powerful British kingdoms in all Albion, thus beginning the period known as the Ennarchy.
The northern part of Northumbria, known as Bernicia, survived intact, centered on the powerful fortresses of Bebbanburh and Eideneburh. It would not again see much success in the south, but over the following centuries it continued to expand north into Gaelic and Pictish territory, thus becoming the forerunner of the modern-day country of England.
In the longer term, the battle resulted in the gradual divergence of the English and Lloegrian languages. Both are descended from dialects of Old English, which prior to the battle formed an unbroken continuum from the Firth of Forth to the Gallic Channel, but the Britannisation of Ebrauc cut direct contact between the two and encouraged two distinct standards to develop in England and Lloegyr. This process, coupled with extensive Irish and Norse influence on English and Frisian, British and Lyonnaise influence on Lloegrian, means that the two now form separate languages which are barely intelligible with one another.