How plausible is it, for instance, to have a quick disintegration of the Western Roman Empire into smaller kingdoms? Let's say the outcome of the Milvian Bridge went differently, and the East abandons Rome to the Germans. How would I be able to stop the Anglo-Saxon invasion of Britain? Would the British Legions just hold their ground, not inviting in the Angles, and elect a new leader (Emperor Arcturus, anyone?) Or would I have to have the Huns move north?