In 121 BC, Rome came into contact with two Celtic tribes (from a region in modern France), both of which they defeated with apparent ease. The Cimbrian War (113–101 BC) was a far more serious affair than the earlier clashes of 121 BC. The Germanic tribes of the Cimbri and the Teutons migrated from northern Europe into Rome's northern territories, and clashed with Rome and her allies. At the Battle of Aquae Sextiae and the Battle of Vercellae both tribes were virtually annihilated, which ended the threat.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): Where did the tribes that were almost annihilated in the Battle of Vercellae hail from?
northern Europe