But what were they to do? It was folly under such circumstances to let the matter come to the decision of arms; for these had already decidedly irrevocably. They might as well attempt to shake the Alps by throwing stones at them as to shake the legions by means of the Celtic bands, whether these might be congregated in huge masses or sacrificed in detail canton after canton. Vercingetorix despaired of defeating the Romans. He adopted a system of warfare similar to that by which Cassivellaunus had saved the insular Celts. The Roman infantry was not to be vanquished; but Caesar's cavalry consisted almost exclusively of the contingent of the Celtic nobility, and was practically dissolved by the general revolt. It was possible for the insurrection, which was in fact essentially composed of the Celtic nobility, to develop such a superiority in this arm, that it could lay waste the land far and wide, burn down towns and villages, destroy the magazines, and endanger the supplies and the communications of the enemy, without his being able seriously to hinder it. Vercingetorix accordingly directed all his efforts to the increase of his cavalry, and of the infantry-archers who were according to the mode of fighting of that time regularly associated with it. He did not send the immense and self-obstructing masses of the militia of the line to their homes, but he did not allow them to face the enemy, and attempted to impart to them gradually some capacity of intrenching, marching, and manoeuvring, and some perception that the soldier is not destined merely for hand-to-hand combat. Learning from the enemy, he adopted in particular the Roman system of encampment, on which depended the whole secret of the tactical superiority of the Romans; for in consequence of it every Roman corps combined all the advantages of the garrison of a fortress with all the advantages of an offensive army[47]. It is true that a system completely adapted to Britain which had few towns and to its rude, resolute, and on the whole united inhabitants was not absolutely transferable to the rich regions on the Loire and their indolent inhabitants on the eve of utter political dissolution. Vercingetorix at least accomplished this much, that they did not attempt as hitherto to hold every town with the result of holding none; they agreed to destroy the townships not capable of defence before attack reached them, but to defend with all their might the strong fortresses. At the same time the Arvernian king did what he could to bind to the cause of their country the cowardly and backward by stern severity, the hesitating by entreaties and representations, the covetous by gold, the decided opponents by force, and to compel or allure the rabble high or low to some manifestation of patriotism.

Even before the winter was at an end, he threw himself on the Boii settled by Caesar in the territory of the Haedui, with the view of annihilating these, almost the sole trustworthy allies of Rome, before Caesar came up. The news of this attack induced Caesar, leaving behind the baggage and two legions in the winter quarters of Agedincum (Sens), to march immediately and earlier than he would doubtless otherwise have done, against the insurgents. He remedied the sorely-felt want of cavalry and light infantry in some measure by gradually bringing up German mercenaries, who instead of using their own small and weak ponies were furnished with Italian and Spanish horses partly bought, partly procured by requisition of the officers. Caesar, after having by the way caused Cenabum, the capital of the Carnutes, which had given the signal for the revolt, to be pillaged and laid in ashes, moved over the Loire into the country of the Bituriges. He thereby induced Vercingetorix to abandon the siege of the town of the Boii, and to resort likewise to the Bituriges. Here the new mode of warfare was first to be tried. By order of Vercingetorix more than twenty townships of the Bituriges perished in the flames on one day; the general decreed a similar self-devastation as to the neighbour cantons, so far as they could be reached by the Roman foraging parties.

According to his intention, Avaricum (Bourges), the rich and strong capital of the Bituriges, was to meet the same fate; but the majority of the war-council yielded to the suppliant entreaties of the Biturigian authorities, and resolved rather to defend that city with all their energy. Thus the war was concentrated in the first instance around Avaricum, Vercingetorix placed his infantry amidst the morasses adjoining the town in a position so unapproachable, that even without being covered by the cavalry they needed not to fear the attack of the legions. The Celtic cavalry covered all the roads and obstructed the communication. The town was strongly garrisoned, and the connection between it and the army before the walls was kept open. Caesar's position was very awkward.

The attempt to induce the Celtic infantry to fight was unsuccessful; it stirred not from its unassailable lines. Bravely as his soldiers in front of the town trenched and fought, the besieged vied with them in ingenuity and courage, and they had almost succeeded in setting fire to the siege apparatus of their opponents. The task withal of supplying an army of nearly 60,000 men with provisions in a country devastated far and wide and scoured by far superior bodies of cavalry became daily more difficult. The slender stores of the Boii were soon used up; the supply promised by the Haedui failed to appear; the corn was already consumed, and the soldier was placed exclusively on flesh-rations. But the moment was approaching when the town, with whatever contempt of death the garrison fought, could be held no longer. Still it was not impossible to withdraw the troops secretly by night and destroy the town, before the enemy occupied it. Vercingetorix made arrangements for this purpose, but the cry of distress raised at the moment of evacuation by the women and children left behind attracted the attention of the Romans; the departure miscarried.

On the following gloomy and rainy day the Romans scaled the walls, and, exasperated by the obstinate defence, spared neither age nor sex in the conquered town. The ample stores, which the Celts had accumulated in it, were welcome to the starved soldiers of Caesar. With the capture of Avaricum (spring of 702), a first success had been achieved over the insurrection, and according to former experience Caesar might well expect that it would now dissolve, and that it would only be requisite to deal with the cantons individually. After he had therefore shown himself with his whole army in the canton of the Haedui and had by this imposing demonstration compelled the patriot party in a ferment there to keep quiet at least for the moment, he divided his army and sent Labienus back to Agedincum, that in combination with the troops left there he might at the head of four legions suppress in the first instance the movement in the territory of the Carnutes and Senones, who on this occasion once more took the lead; while he himself with the six remaining legions turned to the south and prepared to carry the war into the Arvernian mountains, the proper territory of Vercingetorix.

Labienus moved from Agedincum up the left bank of the Seine with a view to possess himself of Lutetia (Paris), the town of the Parisii situated on an island in the Seine, and from this well-secured position in the heart of the insurgent country to reduce it again to subjection. But behind Melodunum (Melun), he found his route barred by the whole army of the insurgents, which had here taken up a position between unassailable morasses under the leadership of the aged Camulogenus. Labienus retreated a certain distance, crossed the Seine at Melodunum, and moved up its right bank unhindered towards Lutetia; Camulogenus caused this town to be burnt and the bridges leading to the left bank to be broken down, and took up a position over against Labienus, in which the latter could neither bring him to battle nor effect a passage under the eyes of the hostile army.