Although the rigour of the season adds to the difficulty of the marches and supplies of provisions (it was in the month of March), he decides upon immediately taking the field. Vercingetorix has just laid siege to Gorgobina, the _oppidium_ of the Boii. These 20,000 Germans, so recently vanquished, preserve the sincere gratitude of a primitive people towards him who has given them lands, instead of selling them for slaves: they remain faithful to the Romans, and face the anger of Vercingetorix and the attacks of revolted Gaul. Cæsar, unwilling that a people who set the example of fidelity should become the victims of their devotedness, marches to their succour. He might go directly to Gorgobina, and cross the Loire at Nevers; but in that case, Vercingetorix, informed of his approach, would have had time to come and dispute the passage. To attempt this by force was a dangerous operation. He leaves at Sens two legions and his baggage, starts at the head of the eight others, and hastens, by the shortest way, to cross the Loire at Gien. He proceeds up the left bank of the river; while Vercingetorix, instead of waiting for him, raises the siege of Gorgobina. He proceeds to meet Cæsar, who beats him at Sancerre in a cavalry encounter, and then marches upon Bourges, without further care for an enemy incapable of arresting him in the open field. The capture of that important town must make him master of the whole country. The Gaulish general confines himself to following by short marches, and burning all the country around, in order to starve the Roman army. The siege of Bourges is one of the most regular and interesting of the war in Gaul. Cæsar opens the trenches, that is, he makes covered galleries which permit him to approach the place, to fill the fosse, and to construct a terrace, a veritable breaching battery, surmounted on each side by a tower. When, with the assistance of his military engines, he has thinned the ranks of the defenders, he assembles his legions under protection of the parallels composed of covered galleries, and by means of the terrace, which equals the elevation of the wall, he gives the assault and carries the place. After the capture of Bourges, he proceeds to Nevers, where he establishes his magazines; then to Decize, to appease the disputes which had arisen, among the Burgundians, from the competition of two claimants to the supreme power. He next divides his army; sends Labienus, with two legions, against the Parisii and their allies; orders him to take the two legions left at Sens; and in person, with the six others, directs his march towards Auvergne, the principal focus of the insurrection. By means of a stratagem, he crosses the Allier at Varennes without striking a blow, and obliges Vercingetorix to retire into Gergovia with all his forces.