For a moment, however, it seemed as if the two peoples, through whose co-operation Etruria saw her very existence put in jeopardy, were about to destroy each other, and the reviving power of Rome was to be trodden under foot by foreign barbarians. This turn of things, so contrary to what might naturally have been expected, the Romans brought upon themselves by their own arrogance and shortsightedness. The Celtic swarms, which had crossed the river after the fall of Melpum, rapidly overflowed northern Italy - not merely the open country on the right bank of the Po and along the shore of the Adriatic, but also Etruria proper to the south of the Apennines. A few years afterwards (363) Clusium situated in the heart of Etruria (Chiusi, on the borders of Tuscany and the Papal State) was besieged by the Celtic Senones; and so humbled were the Etruscans that the Tuscan city in its straits invoked aid from the destroyers of Veii. Perhaps it would have been wise to grant it and to reduce at once the Gauls by arms, and the Etruscans by according to them protection, to a state of dependence on Rome; but an intervention with aims so extensive, which would have compelled the Romans to undertake a serious struggle on the northern Tuscan frontier, lay beyond the horizon of the Roman policy at that time. No course was therefore left but to refrain from all interference. Foolishly, however, while declining to send auxiliary troops, they despatched envoys. With still greater folly these sought to impose upon the Celts by haughty language, and, when this failed, they conceived that they might with impunity violate the law of nations in dealing with barbarians; in the ranks of the Clusines they took part in a skirmish, and in the course of it one of them stabbed and dismounted a Gallic officer. The barbarians acted in this case with moderation and prudence. They sent in the first instance to the Roman community to demand the surrender of those who had outraged the law of nations, and the senate was ready to comply with the reasonable request. But with the multitude compassion for their countrymen outweighed justice towards the foreigners; satisfaction was refused by the burgesses; and according to some accounts they even nominated the brave champions of their fatherland as consular tribunes for the year 364[9], which was to be so fatal in the Roman annals. Then the Brennus or, in other words, the "king of the army" of the Gauls broke up the siege of Clusium, and the whole Celtic host - the numbers of which are stated at 70,000 men - turned against Rome. Such expeditions into unknown land distant regions were not unusual for the Gauls, who marched as bands of armed emigrants, troubling themselves little as to the means of cover or of retreat; but it was evident that none in Rome anticipated the dangers involved in so sudden and so mighty an invasion. It was not till the Gauls were marching upon Rome that a Roman military force crossed the Tiber and sought to bar their way. Not twelve miles from the gates, opposite to the confluence of the rivulet Allia with the Tiber, the armies met, and a battle took place on the 18th July, 364. Even now they went into battle - not as against an army, but as against freebooters - with arrogance and foolhardiness and under inexperienced leaders, Camillus having in consequence of the dissensions of the orders withdrawn from taking part in affairs. Those against whom they were to fight were but barbarians; what need was there of a camp, or of securing a retreat? These barbarians, however, were men whose courage despised death, and their mode of fighting was to the Italians as novel as it was terrible; sword in hand the Celts precipitated themselves with furious onset on the Roman phalanx, and shattered it at the first shock. The overthrow was complete; of the Romans, who had fought with the river in their rear, a large portion met their death in the attempt to cross it; such as escaped threw themselves by a flank movement into the neighbouring Veii. The victorious Celts stood between the remnant of the beaten army and the capital. The latter was irretrievably abandoned to the enemy; the small force that was left behind, or that had fled thither, was not sufficient to garrison the walls, and three days after the battle the victors marched through the open gates into Rome.  Had they done so at first, as they might have done, not only the city, but the state also must have been lost; the brief interval gave opportunity to carry away or to bury the sacred objects, and, what was more important, to occupy the citadel and to furnish it with provisions for the exigency. No one was admitted to the citadel who was incapable of bearing arms--there was not food for all. The mass of the defenceless dispersed among the neighbouring towns; but many, and in particular a number of old men of high standing, would not survive the downfall of the city and awaited death in their houses by the sword of the barbarians. They came, murdered all they met with, plundered whatever property they found, and at length set the city on fire on all sides before the eyes of the Roman garrison in the Capitol. But they had no knowledge of the art of besieging, and the blockade of the steep citadel rock was tedious and difficult, because subsistence for the great host could only be procured by armed foraging parties, and the citizens of the neighbouring Latin cities, the Ardeates in particular, frequently attacked the foragers with courage and success.

Nevertheless the Celts persevered, with an energy which in their circumstances was unparalleled, for seven months beneath the rock, and the garrison, which had escaped a surprise on a dark night only in consequence of the cackling of the sacred geese in the Capitoline temple and the accidental awaking of the brave Marcus Manlius, already found its provisions beginning to fail, when the Celts received information as to the Veneti having invaded the Senonian territory recently acquired on the Po, and were thus induced to accept the ransom money that was offered to procure their withdrawal. The scornful throwing down of the Gallic sword, that it might be outweighed by Roman gold, indicated very truly how matters stood. The iron of the barbarians had conquered, but they sold their victory and by selling lost it.

The fearful catastrophe of the defeat and the conflagration, the 18th of July and the rivulet of the Allia, the spot where the sacred objects were buried, and the spot where the surprise of the citadel had been repulsed - all the details of this unparalleled event - were transferred from the recollection of contemporaries to the imagination of posterity; and we can scarcely realize the fact that two thousand years have actually elapsed since those world-renowned geese showed greater vigilance than the sentinels at their posts. And yet - although there was an enactment in Rome that in future, on occasion of a Celtic invasion no legal privilege should give exemption from military service; although dates were reckoned by the years from the conquest of the city; although the event resounded throughout the whole of the then civilized world and found its way even into the Grecian annals - the battle of the Allia and its results can scarcely be numbered among those historical events that are fruitful of consequences. It made no alteration at all in political relations. When the Gauls had marched off again with their gold - which only a legend of late and wretched invention represents the hero Camillus as having recovered for Rome - and when the fugitives had again made their way home, the foolish idea suggested by some faint-hearted prudential politicians, that the citizens should migrate to Veii, was set aside by a spirited speech of Camillus; houses arose out of the ruins hastily and irregularly - the narrow and crooked streets of Rome owed their origin to this epoch; and Rome again stood in her old commanding position. Indeed it is not improbable that this occurrence contributed materially, though not just at the moment, to diminish the antagonism between Rome and Etruria, and above all to knit more closely the ties of union between Latium and Rome. The conflict between the Gauls and the Romans was not, like that between Rome and Etruria or between Rome and Samnium, a collision of two political powers which affect and modify each other; it may be compared to those catastrophes of nature, after which the organism, if it is not destroyed, immediately resumes its equilibrium. The Gauls often returned to Latium: as in the year 387, when Camillus defeated them at Alba - the last victory of the aged hero, who had been six times military tribune with consular powers, and five times dictator, and had four times marched in triumph to the Capitol; in the year 393, when the dictator Titus Quinctius Pennus encamped opposite to them not five miles from the city at the bridge of the Anio, but before any encounter took place the Gallic host marched onward to Campania; in the year 394, when the dictator Quintus Servilius Ahala fought in front of the Colline gate with the hordes returning from Campania; in the year 396, when the dictator Gaius Sulpicius Peticus inflicted on them a signal defeat; in the year 404, when they even spent the winter encamped upon the Alban mount and joined with the Greek pirates along the coast for plunder, till Lucius Furius Camillus, the son of the celebrated general, in the following year dislodged them - an incident which came to the ears of Aristotle who was contemporary (370-432) in Athens. But these predatory expeditions, formidable and troublesome as they may have been, were rather incidental misfortunes than events of political significance; and their most essential result was, that the Romans were more and more regarded by themselves and by foreigners as the bulwark of the civilized nations of Italy against the onset of the dreaded barbarians - a view which tended more than is usually supposed to further their subsequent claim to universal empire.