All references to Arthur are largely found in histories written by Gildas, Nennius, Geoffrey of Monmouth, and Bede. While these histories are detailed, they have largely lost any historical credibility over the past half millennium. As such, you can examine De Excidio et Conquestu Britanniae, Historia Brittonum, Historia Regum Britanniae, and Ecclesiastical History of the English People to find any common information about an event like what you described, but these and works like them are going to be the only "historical evidence" that you'll likely find about an event involving a fictional person.

The problem, as /u/sprashoo says, is that we actually have very little contemporary evidence about Arthur, and the stories that do survive are not only written much later, but are clearly entirely works of fiction. That doesn't, however, make it entirely impossible to talk about the world the original Arthur (if he existed) would have lived in.

There's a tiny bit of evidence for Arthur being a real person. The Y Gododdin (a poem probably written in the late 6th or 7th century) mentions a heroic leader who 'was not Arthur'. Gildas (Welsh, writing in the late 5th or more probably mid 6th century) mentions the battle of Badon Hill (but not Arthur himself). But that's about it, and these brief mentions tell us no information at all about who this Arthur was or what he did.

For more details, you have to go to sources that are written much later, and these sources have problems that make it difficult to trust the stories they tell. A Welsh chronicler named Nennius gives a bit more information about Arthur in an account probably written in the 9th century, but this account is suspect for two reasons. The first is that it contains a lot of other stories which are rather mythical (for example, he also talks about a 300-year-old holy man). It's not clear that Nennius shared our modern distinctions between 'fact' and 'fiction'. It's also clear (secondly) that his text was written during a time of conflict between Nennius' king and a neighboring Anglo-Saxon kingdom. His Chronicle often reads like wartime propoganda, and the stories it tells about Arthur seem to be pretty heavily spun to make Arthur out to be the folk hero the 9th century Welsh needed to help them beat their contemporary Anglo-Saxon neighbors. This makes it very difficult to separate out the contemporary politics from whatever source material Nennius may have been drawing on; was Arthur originally an enemy of the Saxons? Or is that just Nennius' politics coloring his history? Plus, Nennius actually doesn't give us much detail about Arthur - just a list of battles Arthur supposedly won.

For the good, detailed accounts about Arthur (the Round Table, etc), you have to look to the later middle ages. Geoffrey of Monmouth, writing for Norman royalty in the 12th century (remember: Arthur was probably alive in the 5th or 6th century), used Arthur as a great example of how Britain's best kings were enemies of the Saxons (just like the Norman conquerors were, what a coincidence! ie, this is also politics masquerading as 'factual' history). He claimed to be basing the detailed stories he told about Arthur on lost Welsh histories that he had found... but who knows. Odds are good that he invented the details or, at best, borrowed a bunch of fictional welsh folk stories.

After Monmouth, stories about Arthur explode in popularity, and we get all the details we know so well. Most of our modern ideas about Arthur come from Malory's Morte D'Arthur. But at this point, we've left history far behind, and are dealing with Arthur the literary hero - Malory had about 200 years of late medieval stories to draw on for his version of events, and was writing nearly 1000 years after the real Arthur would have lived, if he were a real person at all.

I'd recommend G. Halsall's Worlds of Arthur if you want to really dig into the early medieval evidence. It's readable, inexpensive (though ILL is your friend, always), and gives a great introduction to how historians and archaeologists study the early middle ages. Halsall concludes that Arthur may well have existed, but that we simply don't have the evidence to say any more than that about him. I think that's a correct, if disappointing, conclusion.

But we can say a lot more about the world he lived in, which is where this gets fun.

We don't have many accounts of what happened during battles in the early middle ages (most accounts say 'there was a battle' - the gory details didn't seem to interest the chroniclers until a somewhat later date), but the kinds of shifting alliances that your question describes seem to have been common, if authors like the 6th century historian Gregory of Tours are to be believed. His 'Ten Books' (later renamed to 'History of the Franks') tells all about the murders, backstabby politics, and intrigues that happened in France during the century when Arthur would have been alive. And it's full of accounts of nobles luring each other into traps, forcing enemies to capitulate, and frequently shifting aliances.

England's politics would have looked a little different from those in France during the sixth century (they seem to have been less centralized / organized), but it's not unlikely that enemy warlords, when captured, were often forced to swear loyalty to their conqueror (and if Gregory's account translates at all to England, these alliances probably ended in just as much betrayal and bloodshed as we find in the late, mythical stories of Arthur). Indeed, it looks like the later 6th century saw a lot of warlords leverage their social positions to become kings of their regions, which must have required some knocking together of heads, and transforming many defeated enemies into loyal followers.

But we lack any specific details for this time period in England (most of the histories of this period are, like Nennius, written much later, and their details are skewed by contemporary - 8th and 9th century - political agendas). So while we can imagine the kind of political world Arthur would have lived in (if he was, in fact, a real figure), we don't know enough to give any specific names and dates for battles.