The people of Fortriu left no surviving indigenous writings and the name they used to describe themselves is unrecorded. They were first documented in the late 4th century by the Roman historian Ammianus Marcellinus, who referred to them in Latin as the Verturiones (or Vecturiones). The Latin root verturio has been connected etymologically by John Rhys with the later Welsh word gwerthyr, meaning "fortress", suggesting that both came from a Common Brittonic root vertera, and implying that the group's name meant "Fortress People". Mallory & Adams saw the name as representing tu(:)rjones, derived from Indo European tur meaning "mighty", with the intensive prefix *wer. A reconstructed form in the Pictish language would be something like *Uerteru.
After the 4th century Fortriu is not explicitly mentioned in documentary sources until 664, but there are indications that Fortriu's later power may have been foreshadowed in the late 6th century. Adomnan's Life of Columba describes the stronghold of the Pictish king Bridei son of Maelchon, who ruled from 554 to 584, as being by the River Ness, in or near to the heartland of Fortriu. Bridei is depicted by Adomnan as overlord of a regulus or "underking" of Orkney, and was separately described by the Northumbrian historian Bede as rex potentissimus or "very powerful king". Irish annals record a "flight" or "migration" of Gaels "before the son of Mailcon" between 558 and 560, suggesting that by then Bridei's power may have been extending into the territory of Cenél Loairn in Dál Riata, at the opposite end of the Great Glen from Fortriu, and Adomnan records a slave girl from Dál Riata at Bridei's court at the time of Columba's visit.
By the end of the 7th century Fortriu had established a dominant position over most or all of the Picts, one of the most significant developments in the history of early medieval Scotland, described by historians as the Verturian Hegemony. The status of Fortriu as a powerful over-kingdom can be seen from the reign of Bridei son of Beli, who was the first king to be explicitly described as "King of Fortriu" in contemporary chronicles, and whose victory over Ecgfrith of Northumbria at the Battle of Dun Nechtain in 685 extended Fortriu's power southward, replacing Northumbrian rule north of the Forth. Bridei had possibly been a sub-king of the Northumbrians at the start of his reign in 671, but began to extend his power with a siege of Dunottar in 680 and an attack on Orkney in 681. As the influence of the kings of Fortriu grew they promoted the idea of the Picts as a single people with a single king, playing a key role in uniting the Picts and establishing a self-conscious Pictish identity.
The continuing power of the kings of Fortriu over the Picts can be seen in the activities of Bridei son of Beli's successors. Bridei son of Derilei and the cleric Curetán of Rosemarkie were the only Pictish signatories to Cáin Adomnáin or "Law of the Innocents" in 697, indicating that Bridei was able to enforce adherence of the Picts as a whole; while Nechtan son of Derilei's church reforms of the 710s were described by Bede as being enacted "throughout all the provinces of the Picts". The kings of Fortriu maintained their control over southern Pictish territories in the 7th and 8th centuries by planting them with loyal Gaelic lords and their military retinues; creating provinces named after leading Gaelic kindreds including Cenél Comgaill in Strathearn, Cenél nÓengusa in Angus and Cenél nGabráin in Gowrie.
A period of instability in Fortriu following the death of Elphin son of Wrad in 780 saw four rulers in quick succession – three from the family of Onuist son of Uurguist – and allowed Dál Riata to reassert its independence. The succession of Constantín son of Uurguist to the kingship of Fortriu in 789 was challenged by the Dál Riatan king Conall mac Taidg, but Constantín proved to be strong leader and reigned through to his death in 820.
Fortriu continued to be recorded into the early 10th century, suggesting a degree of continuity with the earlier period of over-kingship. The Annals of Ulster record the "men of Fortriu" killing the Scandinavian leader Ímar ua Ímair in 904, four years after it had started using the description ri Alban for the King of Alba. The last dated reference to Fortriu in any of the Irish Annals is for 918 in the Fragmentary Annals of Ireland, where the phrases "Men of Fortriu" and "Men of Alba" are treated as synonymous. The Historia Regum Anglorum describes King Aethelstan of England wasting Scotia as far as Dunottar and Wertermorum – the "muir of Fortriu" – in 934, indicating that Fortriu was still recognised at this stage as a reference for features in the landscape.
The complete disappearance of the name Fortriu beyond this point suggests that it fragmented into its successor polities – the provinces of Moray and Ross – during the 10th century. Moray is first recorded in an entry in the Chronicle of the Kings of Alba for the reign of Malcolm I, which lasted from 943 to 954; while Ross first appears in the documentary record in a hagiography of the Scottish-born saint Cathróe of Metz, written in Metz between 971 and 976.
From the 19th century until 2006 most historians believed that the kingdom recorded as Fortriu in the Irish annals lay south of the Mounth in present-day central Scotland, based on the work of E. W. Robertson and W. F. Skene. Robertson, in his 1862 work Scotland under her Early Kings, identified Fortriu as comprising Clackmannanshire, Menteith and west Fife on the left bank of the Forth, arguing that the names of both Fortriu and the medieval deanery of Fothriff derived from an earlier hypothetical *Forthreim, which he translated as "Forth Realm". This argument is based on unsound etymology, however, as Fothriff derives from the Gaelic words foithir and Fib and means "district appended to Fife", while Fortriu is related to the earlier Latin name Verturiones. Skene, in his 3 volume work Celtic Scotland: A History of Ancient Alban, published between 1876 and 1880, identified Fortriu with Strathearn and Menteith, the first province listed in the 12th century document De Situ Albanie, on the basis that a battle recorded by the Chronicle of the Kings of Alba as taking place in Sraith Herenn was also recorded by the Annals of Ulster as the killing of Ímar ua Ímair by the "Men of Fortriu". This argument is also inconclusive, however: Sraith Herenn could refer to either Strathearn in Perthshire, south of the Mounth; or Strathdearn, the valley of the River Findhorn in Moray, north of the Mounth; while the fact that Ímar was killed by the "Men of Fortriu" does not prove that he was killed within the territory of Fortriu. Despite Skene's initial suggestion being tentative, this identification of Fortriu as including western Perthshire became established as a consensus.
There can be little or no doubt then that Fortriu centred on northern Scotland. Other Pictish scholars, such as James E. Fraser are now taking it for granted that Fortriu was in the north of Scotland, centred on Moray and Easter Ross, where most early Pictish monuments are located.
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