When the Romans followed the cares that infest the day, and folded their tents like the Arabs and silently stole away, they left behind that nice long wall right across Britain. The wall had been there for close on three hundred years so, by this time, the wild and dangerous tribes it was built to protect the empire from, were more likely to be in-laws than outlaws. Anyway, the Saxons, moving up from the south were a far more dangerous enemy than your neighbours who lived through one of the gates in the wall.
The Saxons had trouble at home. The bits of Europe they called their own were becoming a little soggy. It was ‘climate change’ you see, probably caused by the methane generated by all those chariot horses used in Roman Circuses, and the sea was encroaching where they would have preferred that it didn’t. Lacking a carbon trading framework the only sensible thing to do was to invade Britain – so they did.
In the south, where a goodly number of Saxon mercenaries had been called in to fight the locals battles for them, it was no problem for the soldiers of fortune to make their fortune usurping their employers. But in the north the inhabitants, as the Romans had found, were a grumpy lot. Urien Rheged, from his base around Carlisle, had put together a coalition of the grumpiest he could find and they held the Saxons back for nearly a hundred years.
Then, as with most historical figures, he was assassinated, and his son, who took over, just didn’t have that je ne sais quoi.