You can’t make this stuff up
Europe as we once knew it will be lost anyway, with millions of a different culture and ethnic background begin to overwhelm the “aboriginal” Europeans. Some due to the flight of the French, Germans, Dutch and various Scandinavians to other lands and climes. Some due to being “aged out”, as the reproduction rate falls to zero and effectively becomes negative. Some due to intermarriage and obliteration of the original culture, And some due to, well, murder and pillage by the invasive horde.
The Roman Empire was overrun by the Goths, but the Roman culture remained and spread its influence throughout the northern reaches of Europe, with even a brief resurrection of the glory that was Rome in the “Holy Roman Empire”, under Charlemagne, though it was neither holy, Roman, nor an empire in the recognized sense.
Surrender on the installment plan, with a big balloon payment not too far up into the future.