Britain was once a Roman province. A few centuries passed and it was no longer a Roman province. The problem is that no one was around to write down what happened. Or if there were such scribes their works have mostly been lost.
A fellow named Gildas wrote, in Latin, a treatise called De Excidio et Conquestu Britanniae which means “On the Ruin and Conquest of Britain.” It was composed, most likely, in the early 6th century, perhaps about AD 510-530. This is one of the only written sources about the decline and fall of Roman rule in Britain.
Hadrian’s Wall was built in the second century. The start date is usually given as 122 AD. Early in the fifth century, around 410, there is evidence of Britons seeking help from the Emperor but being denied. By the end of the century the Roman province of Britannia would cease to be. What happened? And why? There is a lot of scholarship but not a lot of answers. It’s an interesting time precisely because we know so little. The legend of King Arthur emerged from this era and those stories still generate debate about their historicity.
The folks at GMT Games have stepped into this morass and offered their own take:

This is a game. It’s a very complicated game with a 72-page rulebook. It attempts to model the conflicts between the Romans, Romanized Britons, and the barbarians (Saxons and Celts) who raided and invaded the province of Britannia.
This game appealed to me precisely because it’s a time in history where most of the information has been lost. There are plenty of wargames out there that model famous battles where the movement of every battalion is well known. Think about the Battle of Waterloo, for example. You can play lots of games where you are Napoleon or Wellington or whatnot and you can re-create famous moments in history.
But that’s no fun. We know how those turn out. Pendragon is interesting because we know how things turned out, but not how they got there.
I bought this game when it came out several years ago and have struggled with it. It’s got a lot going on. I boxed it back up and put it away after several frustrating attempts to learn to play properly. I’ve decided to give it another try so I set it up again and I’m going to run through the practice scenario. Then I’ll play against the “bots” which are mostly just flowcharts. There are four factions (Dux, Civitates, Saxons, Scotti) and you pick one to face off against the other three. Or you play “barbarians” (Saxons and Scotti) against “Britons” (Dux and Civitates). At some point in the game the alliance between the Dux (Imperial troops) and the Briton landlords (Civitates) breaks down and the factions enter into open conflict. The Celtic raiders (Scotti) and their Continental counterparts (Saxons) don’t work together, and are hostile to each other, but they do have the same goals—gaining plunder and ultimately creating settlements on the island.
The game designers provide a lot of background and discussion about how they modeled the historical situation. Here’s a sample of some of their stuff:

I like this sort of thing. I’m wondering if more students would like history if it was presented dynamically and could be modeled with a game. This diagram suggest movement and flow, not the static collection of facts which (unfortunately) history class is often seen as.
Here’s more:

Wargaming is part of all professional military schools. The US Navy wargamed, in advance, every scenario they encountered in WWII against Japan. Except for the kamikazes, that is. Otherwise they had worked out “if they do this, we’ll do that” or “if we do this, they’ll do that” for dozens of possibilities and so were prepared, strategically, for the war’s challenges.
This kind of wargaming is not that, of course. Pendragon is about conflict. And conflict between peoples, states, and institutions is part of life. Perhaps if we learn about previous conflicts, and how to model them, we can understand contemporary conflicts better. And if we understand something we might improve on it.

I’ll let you know how it goes.