The mainland of Italy proper, south of the Apennines, enjoyed profound peace after the fall of Tarentum: the six days' war with Falerii (513) was little more than an interlude. But towards the north, between the territory of the confederacy and the natural boundary of Italy - the chain of the Alps - there still extended a wide region which was not subject to the Romans. What was regarded as the boundary of Italy on the Adriatic coast was the river Aesis immediately above Ancona. Beyond this boundary the adjacent properly Gallic territory as far as, and including, Ravenna belonged in a similar way as did Italy proper to the Roman alliance; the Senones, who had formerly settled there, were extirpated in the war of 471-2[12], and the several townships were connected with Rome, either as burgess-colonies, like Sena Gallica[13], or as allied towns, whether with Latin rights, like Ariminum[14], or with Italian rights, like Ravenna. On the wide region beyond Ravenna as far as the Alps non-Italian peoples were settled. South of the Po the strong Celtic tribe of the Boii still held its ground (from Parma to Bologna); alongside of them, the Lingones on the east and the Anares on the west (in the region of Parma) - two smaller Celtic cantons presumably clients of the Boii - peopled the plain. At the western end of the plain the Ligurians began, who, mingled with isolated Celtic tribes, and settled on the Apennines from above Arezzo and Pisa westward, occupied the region of the sources of the Po. The eastern portion of the plain north of the Po, nearly from Verona to the coast, was possessed by the Veneti, a race different from the Celts and probably of Illyrian extraction. Between these and the western mountains were settled the Cenomani (about Brescia and Cremona) who rarely acted with the Celtic nation and were probably largely intermingled with Veneti, and the Insubres (around Milan). The latter was the most considerable of the Celtic cantons in Italy, and was in constant communication not merely with the minor communities partly of Celtic, partly of non-Celtic extraction, that were scattered in the Alpine valleys, but also with the Celtic cantons beyond the Alps. The gates of the Alps, the mighty stream navigable for 230 miles, and the largest and most fertile plain of the then civilized Europe, still continued in the hands of the hereditary foes of the Italian name, who, humbled indeed and weakened, but still scarce even nominally dependent and still troublesome neighbours, persevered in their barbarism, and, thinly scattered over the spacious plains, continued to pasture their herds and to plunder. It was to be anticipated that the Romans would hasten to possess themselves of these regions; the more so as the Celts gradually began to forget their defeats in the campaigns of 471 and 472 and to bestir themselves again, and, what was still more dangerous, the Transalpine Celts began anew to show themselves on the south of the Alps.

In fact the Boii had already renewed the war in 516, and their chiefs Atis and Galatas had - without, it is true, the authority of the general diet - summoned the Transalpine Gauls to make common cause with them. The latter had numerously answered the call, and in 518 a Celtic army, such as Italy had not seen for long, encamped before Ariminum. The Romans, for the moment much too weak to attempt a battle, concluded an armistice, and to gain time allowed envoys from the Celts to proceed to Rome, who ventured in the senate to demand the cession of Ariminum - it seemed as if the times of Brennus had returned. But an unexpected incident put an end to the war before it had well begun. The Boii, dissatisfied with their unbidden allies and afraid probably for their own territory, fell into variance with the Transalpine Gauls. An open battle took place between the two Celtic hosts; and, after the chiefs of the Boii had been put to death by their own men, the Transalpine Gauls returned home. The Boii were thus delivered into the hands of the Romans, and the latter were at liberty to expel them like the Senones, and to advance at least to the Po; but they preferred to grant the Boii peace in return for the cession of some districts of their land (518). This was probably done, because they were just at that time expecting the renewed outbreak of war with Carthage; but, after that war had been averted by the cession of Sardinia, true policy required the Roman government to take possession as speedily and entirely as possible of the country up to the Alps. The constant apprehensions on the part of the Celts as to such a Roman invasion were therefore sufficiently justified; but the Romans were in no haste. So the Celts on their part began the war, either because the Roman assignations of land on the east coast (522), although not a measure immediately directed against them, made them apprehensive of danger; or because they perceived that a war with Rome for the possession of Lombardy was inevitable; or, as is perhaps most probable, because their Celtic impatience was once more weary of inaction and preferred to arm for a new warlike expedition. With the exception of the Cenomani, who acted with the Veneti and declared for the Romans, all the Italian Celts concurred in the war, and they were joined by the Celts of the upper valley of the Rhone, or rather by a number of adventurers belonging to them, under the leaders Concolitanus and Aneroestus[15]. With 50,000 warriors on foot, and 20,000 on horseback or in chariots, the leaders of the Celts advanced to the Apennines (529). The Romans had not anticipated an attack on this side, and had not expected that the Celts, disregarding the Roman fortresses on the east coast and the protection of their own kinsmen, would venture to advance directly against the capital. Not very long before a similar Celtic swarm had in an exactly similar way overrun Greece. The danger was serious, and appeared still more serious than it really was. The belief that Rome's destruction was this time inevitable, and that the Roman soil was fated to become the property of the Gauls, was so generally diffused among the multitude in Rome itself that the government reckoned it not beneath its dignity to allay the absurd superstitious belief of the mob by an act still more absurd, and to bury alive a Gaulish man and a Gaulish woman in the Roman Forum with a view to fulfil the oracle of destiny. At the same time they made more serious preparations. Of the two consular armies, each of which numbered about 25,000 infantry and 1100 cavalry, one was stationed in Sardinia under Gaius Atilius Regulus, the other at Ariminum under Lucius Aemilius Papus. Both received orders to repair as speedily as possible to Etruria, which was most immediately threatened. The Celts had already been under the necessity of leaving a garrison at home to face the Cenomani and Veneti, who were allied with Rome; now the levy of the Umbrians was directed to advance from their native mountains down into the plain of the Boii, and to inflict all the injury which they could think of on the enemy upon his own soil. The militia of the Etruscans and Sabines was to occupy the Apennines and if possible to obstruct the passage, till the regular troops could arrive. A reserve was formed in Rome of 50,000 men. Throughout all Italy, which on this occasion recognized its true champion in Rome, the men capable of service were enrolled, and stores and materials of war were collected.