The migration of the Cimbri and the Teutons from Jutland across Central and Western Europe and into Northern Italy.
The Cimbri and Teutons were the first Germanic people mentioned in history, although the term Teuton itself may in fact be of Celtic origin.

They left their Northern European homeland for unknown reasons and made their track south in search for new lands to settle.
But the Roman Republic, gaining in strength and power across the Mediterranean saw them as a threat and intervened, being beaten back by the Germans multiple times before defeating them.