There were only two roads by which they could leave Helvetia; one crossed the country of the Sequani, the entrance to which was defended by a narrow and difficult defile, situated between the Rhone and the Jura (the _Pas-de-l'Ecluse_), and where the wagons could with difficulty pass one at a time. As this defile was commanded by a very lofty mountain, a handful of men was sufficient to prevent the access. The other road, less contracted and more easy, crossed the Roman province, after having passed the Rhone, which separated the Allobroges from the Helvetii, from Lake Léman to the Jura. Within this distance the river was fordable in several places.[158] At Geneva, the extreme limit of the territory of the Allobroges towards Helvetia, a bridge established a communication between the two countries. The Helvetii decided on taking the most convenient road; they reckoned, moreover, on the co-operation of this neighbouring people, who, but recently subjugated, could have but doubtful sympathies for the Romans.[159]
II. Cæsar, learning that the Helvetii intended to pass through the Roman province, left Rome hastily in the month of March, hurried by forced marches into Transalpine Gaul, and, according to Plutarch, reached Geneva in eight days.[160] As he had in the province only a single legion, he ordered a levy of as many men as possible, and then destroyed the bridge of Geneva. Informed of his arrival, the Helvetii, who were probably not yet all assembled, sent their men of noblest rank to demand a passage through the country of the Allobroges, promising to commit no injury there; they had, they said, no other road to quit their country. Cæsar was inclined to refuse their demand at once, but he called to mind the defeat and death of the Consul L. Cassius; and wishing to obtain time to collect the troops of which he had ordered the levy, he gave them hopes of a favourable reply, and adjourned it to the Ides of April (8th of April). By this delay he gained a fortnight; it was employed in fortifying the left bank of the Rhone, between Lake Léman and the Jura.[161] If we estimate at 5,000 men the legion which was in the province, and at 5,000 or 6,000 the number of soldiers of the new levies, we see that Cæsar had at his disposal, to defend the banks of the Rhone, about 10,000 or 11,000 infantry.[162]