Several of Northumbria's subject nations had rebelled in recent years, leading to a number of large-scale battles against the Picts, Mercians and Irish, with varied success.

After sieges of neighbouring territories carried out by the Picts, Ecgfrith led his forces against them, despite advice to the contrary, in an effort to reassert his suzerainty over the Pictish nations.

The Annals of Tigernach record a siege of "Etain" in 638,[1] which has been interpreted as Northumbria's conquest of Eidyn (Edinburgh) during the reign of Oswald, marking the annexation of Gododdin territories to the south of the River Forth.

Soon after, the Picts rose in rebellion against Northumbrian subjugation at the Battle of Two Rivers, recorded in the 8th century by Stephen of Ripon, hagiographer of Wilfrid.

[16] While none of the historical sources explicitly state Ecgfrith's reason for attacking Fortriu in 685, the consensus is that it was to reassert Northumbria's control over the Picts.

Additional detail is given in the Irish annals of Ulster and Tigernach, and by the early Welsh historian Nennius in his Historia Brittonum (written around a century later).

The Picts, led by Bridei, feigned retreat and drew Ecgfrith's Northumbrian force into an ambush on Saturday, 20 May 685 at a lake in mountains near Duin Nechtain.

[17] The Irish Annals have provided perhaps the most useful resource for identifying the battle site, giving the location as Dún Nechtain, 'Nechtan's Fort', a name that has survived into modern usage in two separate instances.

[28] Earlier local tradition, related by Headrick in the Second Statistical Account, claimed that the site was the location of the Battle of Camlann, where King Arthur fought Mordred.

[34] In a paper published in 2006, historian Alex Woolf gives a number of reasons for doubting Dunnichen as the battle site, most notably the absence of "inaccessible mountains" in mid-Angus.

He makes a case for an alternative site at Dunachton in Badenoch (grid reference NH820047), on the north-western shore of Loch Insh, which shares Dunnichen's toponymical origin of Dún Nechtain.

[21] James Fraser of Edinburgh University suggests that, while it is too early to discount Dunnichen as a potential battle site, locating it there requires an amount of "special pleading" that Dunachton does not need.