Moreover the considerable Roman cavalry, numbering almost 4000 horse, proved utterly untrustworthy--which doubtless admitted of explanation, for they consisted almost wholly of Celtic horsemen, especially of the mounted retainers of the Haedui, under the command of Dumnorix the well-known enemy of the Romans, and Caesar himself had taken them over still more as hostages than as soldiers.

There was good reason to believe that a defeat which they suffered at the hands of the far weaker Helvetic cavalry was occasioned by themselves, and that the enemy was informed by them of all occurrences in the Roman camp.

The position of Caesar grew critical; it was becoming disagreeably evident, how much the Celtic patriot party could effect even with the Haedui in spite of their official alliance with Rome, and of the distinctive interests of this canton inclining it towards the Romans; what was to be the issue, if they ventured deeper and deeper into a country full of excitement, and if they removed daily farther from their means of communication?
The armies were just marching past Bibracte (Autun), the capital of the Haedui, at a moderate distance; Caesar resolved to seize this important place by force before he continued his march into the interior; and it is very possible, that he intended to desist altogether from farther pursuit and to establish himself in Bibracte.

But when he ceased from the pursuit and turned against Bibracte, the Helvetii thought that the Romans were making preparations for flight, and now attacked in their turn.
Battle at Bibracte Caesar desired nothing better.

The two armies posted themselves on two parallel chains of hills; the Celts began the engagement, broke up the Roman cavalry which had advanced into the plain, and rushed on against the Roman legions posted on the slope of the hill, but were there obliged to give way before Caesar's veterans.
When the Romans thereupon, following up their advantage, descended in their turn to the plain, the Celts again advanced against them, and a reserved Celtic corps took them at the same time in flank.
The reserve of the Roman attacking column was pushed forward against the latter; it forced it away from the main body towards the baggage and the barricade of waggons, where it was destroyed.
The bulk of the Helvetic host was at length brought to give way, and compelled to beat a retreat in an easterly direction--the opposite of that towards which their expedition led them.