The Battle of Dun Nechtain is fought between a Pictish army under King Bridei III and the invading Northumbrians under King Ecgfrith, who are decisively defeated.

The Battle of Dun Nechtain or Battle of Nechtansmere was fought between the Picts, led by King Bridei Mac Bili, and the Northumbrians, led by King Ecgfrith, on 20 May 685.

The Picts were a group of peoples in what is now Scotland north of the Firth of Forth, in the Early Middle Ages. Where they lived and details of their culture can be gleaned from early medieval texts and Pictish stones. The name Picti appears in written records as an exonym from the late third century AD. They are assumed to have been descendants of the Caledonii and other northern Iron Age tribes. Their territory is referred to as "Pictland" by modern historians. Initially made up of several chiefdoms, it came to be dominated by the Pictish kingdom of Fortriu from the seventh century. During this Verturian hegemony, Picti was adopted as an endonym. This lasted around 160 years until the Pictish kingdom merged with that of Dál Riata to form the Kingdom of Alba, ruled by the House of Alpin. The concept of "Pictish kingship" continued for a few decades until it was abandoned during the reign of Caustantín mac Áeda.

Bridei son of Beli, died 692 was king of Fortriu and of the Picts from 671 until 692. His reign marks the start of the period known to historians as the Verturian hegemony, a turning point in the history of Scotland, when the uniting of Pictish provinces under the over-kingship of the kings of Fortriu saw the development of a strong Pictish state and identity encompassing most of the peoples north of the Forth.

Northumbria was an early medieval kingdom in what is now Northern England and South Scotland.