Cæsar followed the Helvetii, and sent before him all his cavalry to watch their march. These, too eager in the pursuit, came to blows with the enemy's cavalry in a position of disadvantage, and experienced some loss. Proud of having repulsed 4,000 men with 500 horsemen, the Helvetii became sufficiently emboldened to venture sometimes to harass the Roman army. But Cæsar avoided engaging his troops; he was satisfied with following, day by day, the enemies at a distance of five or six miles at most (about eight kilomètres), opposing the devastations they committed on their passage, and waiting a favourable occasion to inflict a defeat upon them.
The two armies continued their march extremely slowly, and the days passed without offering the desired opportunity. Meanwhile, the provisionment of the Roman army began to inspire serious uneasiness; wheat arrived no longer by the Saône, for Cæsar had been obliged to move from it in order to keep up with the Helvetii. On another hand, the Ædui delayed, under vain pretexts, sending the grain which they had promised. The harvest, too, was not yet ripe, and even forage failed. As the day for distribution approached, Cæsar convoked the Æduan chiefs, who were numerous in his camp, and overwhelmed them with reproaches. One of them, Liscus, occupied in his country the supreme magistracy, under the name of _vergobret_; he denounced Dumnorix, the brother of Divitiacus, as opposing the sending of provisions; it was the same Dumnorix who had heretofore secretly negotiated the passage of the Helvetii across the country of the Sequani, and who, placed at the head of the Æduan contingent, had, in the last combat, by retreating with his men, led to the flight of the whole body of the cavalry. Cæsar sent for Divitiacus, a man devoted to the Roman people, and revealed to him the culpable conduct of his brother, which merited an exemplary punishment. Divitiacus expressed the same opinion, but, in tears, implored the pardon of Dumnorix. Cæsar granted it to him, and contented himself with placing him under surveillance. It was, indeed, good policy not to alienate the Æduan people by any excessive severity against a man of power among them.
The Helvetii, after advancing northward as far as Saint-Vallier, had turned to the west to reach the valley of the Loire. Arrived near Issy-l'Evêque, they encamped on the banks of a tributary of the Somme, at the foot of Mount Tauffrin, eight miles from the Roman army. Informed of this circumstance, Cæsar judged that the moment had arrived for attacking them by surprise, and sent to reconnoitre by what circuits the heights might be reached. He learnt that the access was easy, and ordered Labienus to gain, with two legions, the summit of the mountain by bye-roads, without giving alarm to the enemy, and to wait till he himself, marching at the head of the four other legions, by the same road as the Helvetii, should appear near their camp; then both were to attack them at the same time. Labienus started at midnight, taking for guides the men who had just explored the roads. Cæsar, on his part, began his march at two o'clock in the morning (_de quarta vigilia_),[180] preceded by his cavalry. At the head of his scouts was P. Considius, whose former services under L. Sylla, and subsequently under M. Crassus, pointed him out as an experienced soldier.