Sometimes also known as the Battle of Dunnichen, fought between the forces of Northumbria and Pictavia on the 20th May 685.
In the spring of 685 the Northumbrian king, Ecgfrith, launched a full scale invasion of the kingdom of the Picts or Pictavia.
Pictavia had of course, been "reduced to slavery and remained subject to the yoke of captivity" (1) since at least their defeat in 672 (that is, forced to pay tribute to Northumbria). We do not know precisely why Ecgfrith chose to invade the north at that time. We know from Bede, that he was advised against it; perhaps the Picts had been slow in paying the required tribute or perhaps Ecgfrith decided it was time to follow up the victory of 672, and more comprehensively integrate the subject province of the Picts withih his kingdom of Northumbria.
The native Picts naturally raised an army to oppose Ecgfrith, and under their king Bruide son of Bili, they selected the marshy area of Nechtansmere to make their stand.
Bruide adopted the stratagem of splitting his forces in two; one half he secreted in the nearby mountains, whilst the other half went to confront the enemy. After initially engaging the Northumbrians they ran away, drawing the enemy into the narrow mountain passes. Here the Northumbrians were ambushed by the Picts and comprehensively slaughtered. Ecgfrith was killed and most of his army destroyed.
The result of the battle was a disaster for Northumbria, it effectively ended their territorial ambitions beyond the Forth and not only freed the Picts themselves from the Northumbrian yoke but also the neighboroung northern kingdoms of Dalriada and Strathclyde. As Bede wrote;
From that time the hopes and strength of the English crown "began to waver and retrograde"; for the Picts recovered their own lands, which had been held by the English and the Scots that were in Britain, and some of the Britons their liberty, which they have now enjoyed for about fortysix years
The battle is one of key importance in the history of Scotland. Although the southern territory of Lothian and Din Eidyn itself (that is Edinburgh), remained in Northumbrian hands for another couple of centuries, by curbing the northward expansion of Northumbria it freed the north of Britain from English domination and created an historical breathing space that allowed the kingdoms of Pictavia and Dalriada to eventually unite.
Had Ecgfrith won the battle it is unlikely there would ever have been a nation called Scotland.
(1) According to Eddius Stephanus; the subjucation of the Picts to Northumbria probably date dback to the days of Oswiu.
(2) Nechtansmere is a flat marshy area overlooked by the hills surrounding Dunichen which lies a few miles east of Forfar in the county of Angus
(3) "Forty-six years" being a reference to the Northumbrian capture of Din Eidyn in the siege of 638