On 20 May, 685, Ecgfrith, king of Northumbria, was killed and his army defeated by a smaller force of Picts under their king, Bridei. The battle of Nechtansmere, or Dùn Nechtain, radically altered the balance of power in northern Britain.
English Northumbria had been in the ascendancy and ruled over southern parts of Pictland; with this blow, it was forced to retreat, permanently, south of the Forth.
From this point, the Pictish kingdom which would form the core of the later kingdom of Scots grew to become the major force in the north. Culturally and linguistically the battle was a major turning point. For all its significance, it was not a simple "us and them" battle. We don't know their exact relationship, but Ecgfrith and Bridei were close cousins. Ecgfrith was not an "English" king but, rather, ruler of an English kingdom well established in the Lothians and Borders. This was, effectively, a battle between kings in what is now Scotland.
Both kings had further cultural connections: Bridei was probably son of the British king of Dumbarton. Ecgfrith's father and uncles had been raised in Gaelic Argyll and Ireland, and Pictland. The battle's outcome was celebrated by Gaelic poets.
Scotland in the early middle ages is anything but a simple place, politically, culturally or linguistically. We search in vain for some "ur-Scot" who can bear the fortunes of an uncomplicated national past. According to Bede, writing in the post-Nechtansmere world of 731: "There are in Britain … five languages and four nations, English, British, Gaels and Picts." For Britain, read "Scotland". Bede was a northerner and the multicultural nature of Britain was obvious to him.
Scotland in the early middle ages was possibly the most linguistically diverse region in Europe, also containing, a bit later, Old Norse. If only two of these languages (English/Scots and Gaelic) made it into the modern period, all left their mark on the landscape. We walk through this rich linguistic past in our place-names: Glasgow, Edinburgh, Perth, Kirkwall.
In 1998 I edited The Triumph Tree, an anthology of poetry from all of Scotland's languages before 1350. It was a radical thing to do, "Scottish literature" having had a national foundation myth in Barbour's Bruce, and is still somewhat controversial.
All of Scotland's early literature is shared literature, with Wales, Ireland, England, Iceland and Norway. Early Scottish literature is inherently "British" in this wider sense; perhaps we could call it "archipelagic", a literature of these islands more than of any polity.
Scotland's past holds out an opportunity to look to our shared literatures and our multiple underlying cultures, and find definition in openness and plurality. But we must confront conflict and violence too. Both Gaelic and English (and its descendant language, Scots) were expansionist languages and cultures in the middle ages, at the expense of other cultures. Intense anti-Gaelic rhetoric is a feature of the late medieval and early modern period and persists to this day. That said, we live in a good moment. Attitudes towards Gaelic, Scots and recent newcomer languages in Scotland have become more positive. We have been lucky with our devolved governments, which have been largely supportive of the plurality of cultures of Scotland.
But establishment and popular attitudes have not always been thus, and may not remain. Scotland could provide a better model of what it means to be "British". If Scotland becomes independent, it is a model that must be treasured lest some reductionist Scottishness edges out Scotland's other cultures, just as happens with elephantine Englishness in Britain. In or out of the Union, Scotland has no easily resolved inherent ethnicity. It was and is a country of many languages and cultures. We contradict ourselves. We contain multitudes.