Arminius

Germanic chieftain of the Cherusci tribe — who, despite having served in the Roman auxiliary forces, organized and led the Germanic tribes to destroy three Roman legions (20,000 men) at the Battle of the Teutoburg Forest in 9 AD — ending Roman expansion east of the Rhine.

Stops Roman expansion into Germania permanently — the Teutoburg Forest ambush (three legions annihilated) was the worst Roman military defeat since Cannae; Augustus reportedly paced his palace crying 'Varus, give me back my legions!' — and the Rhine became the permanent Roman border.