The Romans controlled about 1/4 of what is now Germany, and about 1/2 of Austria for a little over 150 years. Withdrawing from those two areas, around 300 AD is what triggered the fall of the Roman Empire. The Goths came from Austria, where they had been Roman Citizens, and ran to Italy for protection when the Huns came in. The Vandals invaded what is now France from Roman Germany to escape from the same Huns.
It was all caused by reducing the Army to pay for more social programs (Bread and Circuses). When the emergency came, the Army that had protected Rome and it's empire for a thousand years was just not there any more. These moves included pulling out of Britain. refugees from Britain became the Brittany region of modern France.