On the historical ups and downs of Britain as a prize...
The trouble with this is WHY would they want to invade England? The country has no resources that are not already available on the mainland and was not the prize it was in 1000 onwards (tax revenue, good farm land etc.). The Saxons came becuse they where there already - having been imported by the Romans
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Motive is very interesting and important question - why was Britain a prize in the 1st century BC (to the Romans), a prize to the Saxons again in the 5th and 6th century, and a prize to the Normans in 1000 AD, but not a prize in the intervening period where I placed the AH challenge in the OP?
At least culturally, Britain and Ireland were not worthless during the Dark Ages. Ireland was a haven of literacy and missionary activity, and after its conversion to Christianity, Englad was a major center of missionary activity into Germany. The relationship is not perfect, but the ability to support a higher level monastic culture, maintaining literacy and sending out missionaries far and wide, suggests that the base territory is agriculturally prosperous enough to support all this activity.
I believe during the Dark Ages in the 7th, 8th and 9th centuries, there were periods where Northumbria was more advanced than much of western Europe, and Mercia and Wessex later had periods of cultural effloressence.
---now I suppose in relative terms, England's taxability might have ebbed to a low point in the 8th, 9th and 10th centuries due to a change to lower average temperatures, shorter growing seasons, plague based deopopulation and accompanying reclamation of more farmland by forest or swamp? Is there archaeological evidence this happened? Likewise, by the year 1000 AD, I believe the climate was getting a bit warmer and population had probably expanded, so this probably resulted in more forest and swampland being cleared for farmland and grazing, with a longer growing season, thus making the territory more taxable. This would of course make your argument fundamentally correct and go a long way to explain England's freedom from continental invasion from 666 to 1066.
Does this interpretation track with archaeology, written history and natural history?----
Now Charlemagne sent forces far afield past the Pyrenees and Alps, and into Bavaria, Bohemia and Saxony. The latter territories were probably not any more taxable than contemporary England. In terms of logistics, invading England probably would not have been much harder with the technology of the time. Yet Charlemagne likely had defensive and religious motives in Central Europe there that would not have applied to England.
As for the invasion of Europe, this was when Britany in France and Britonia were created, does that count?
Well I'm not sure, but I think Brittany was getting established prior to 666 AD. How violent a migration was it? Is there much evidence of local population displacement, or historical chronicles of accompanying battles?
If so a larger number of immigrants from England (fleeing the Saxon take over) could make this more important players in the local / national scene.
Very interesting point and question. In fact as soon as I posted this what if, I was thinking of this as my next AH Challenge, "Maximum Brittany".
Could we get the Bretons expanding down to Bordeaux, or up to Paris? Is it ASB for France to be established around a Breton core by the end of the Middle Ages?