Beware with using the term "Unite" for a barbarian faction and its supposed geographic "natural border". There was no supra-national identity between the tribes that made them part of a single "Gaul" or "Thrace", so it is impossible that these can be more than just geographic terms. The correct wording would be akin to "subjugating" or "integrating" all other tribes into one dominant confederacy or a stronger tribe, e.g. the Arverni.