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Do you have reason to believe that it is actually in some way based on an actual war? A quick look at Wikipedia says that it is based on loosely on Arthurian fantasy, combined with Norse mythology, and Celtic legends. The game itself is set after King Arthur's death, in which his kingdom has been split into three realms, the ones you stated, which are now warring with each other.
Sounds like actual history - between the Roman exit from Britain in around 450 AD and the final conquest of Wales by the English in the 1100's, you could be describing the general back-and-forth struggling between Scandinavia, Anglo-Saxon England and the main Celtic/Gaelic countries. For instance, the "Welsh" Kingdom that existed in what is now the border country between England and Scotland lasted for a surprisingly long time before it was over-run (no trace remains except in ruins and local dialect English) but you have this long eight hundred or so year old history composed of wars, constantly shifting borders and at least four different players. If nothing else, you can set any semi-historical or mythological scenario you like, in a lot of mediums, into this place and time-span. try Bernard Cornwell's The Saxon Stories series for an immersion into the genuine history of the period and insights into the minds of the principals.
Edited by AgProv Elderly curmudgeon and awkward person. Professional old fart.

Can someone tell me the real war that Dark Age of Camelot has as its main PVP? The war is between Britain, Ireland, And Norway.