An abiding preoccupation of the Roman mind remained vivid throughout the history of the city: this was the vulnerability of this Italic peninsula to invasion, especially from the north. The impact of frequent Celtic invasions was a powerful factor in the formation of the attitude with which the Romans apprehended real or imagined menace from foreign peoples. Acts of bad faith which Roman commanders in Gaul and Spain sometimes committed against Celtic adversaries may be seen not entirely as instances of heartless finesse applied against innocently heroic tribesmen: they were at least partly the results of desperate fear arising from a history of suffering at the hands of invading hordes. The tumultus, that declaration of a state of defensive anxiety and preparation against a barbarian invasion, is a characteristically Roman formalisation of a terror that was never completely laid to rest. That fear, even at the time of Rome’s abundant power in the period of the early Principate, prompted efforts to stabilise barbarian frontiers with Rome thousands of miles away from Italy and to base these limits on natural features which would obstruct intending invaders. In the case of the invading Celts, the Alps had proved to be ineffective barriers in themselves. We have noted the tradition that Ambigatus sent out his nephews with armies of conquest towards Italy and towards the Hercynian forest.
Livy’s chronology (5.34-5) wrongly places this movement of Celtic warriors in the time of the elder Tarquin’s kingship in Rome: more probably it relates to much later events in the fifth and fourth centuries BC (Sot. 5). Archaeological evidence suggests the presence of wandering groups of Celts in the Po valley in the fifth century BC (Sot. 6). These were the corm of the army that was to ransack Rome in 390 BC, not only inflicting terrible