The first expansion pack for Rome: Total War, Barbarian Invasion takes the action forward in time 349 years after the end of the original game. Beginning in 363 AD and ending in 476 AD (although the player can choose to continue beyond this point if they see fit), it depicts the great migrations of the Germanic and steppe peoples (notably the Huns) and simulates the religious tension of the time as the three religions of Christianity, Zoroastrianism and Paganism fight for dominance.

Rome has become an empire, grown and split into the Eastern and Western Roman Empires, ruled from Rome and Constantinople. The various regions of the Empire represent the factions of the original game, all of which have been absorbed into the Empire. The ‘Barbarians’ in the title take the form of the Huns, the Goths, the Vandals, the Franks, the Burgundians and the Lombards and it is their respective unification and resulting strength that allows them to invade the Empire itself, acting as the catalyst for the action of the campaign.