alternatehistory.com

Sometime before September 1939, have a foreign power successfully invade and conquer the UK.

The POD can be anytime after 11:00pm, November 11, 1918, so:
•WWI is over
•The Germans have been defeated, and they will soon have strict limits imposed on them at the negotiating table
•Russia is in the middle of a civil war, which the Allies are intervening in

So, would any countries be up to the task in the 20s or 30s?
•Where would the invasion stage from? The obvious point is mainland Europe, across the Channel or North Sea, but anyone invading from there would have to also dominate mainland Europe. Other options are Ireland or Norway, but I think the UK would intervene long before a foreign power could build up forces in Ireland, and Norway is a bit of a swim
•Would the invader have to decisively
destroy the Royal Navy, or could the RN be kept at bay without being destroyed?

The first options that come to mind are a France that has a falling out with the UK (possibly after a revolution?) Or an aggressive USSR that's hostile to the West after the Allied intervention in the Russian Civil War
Top