After Caesar's victory at Bibracte, Gallic leaders asked Caesar for protection against the German (Suebi) king Ariovistus. They said he had brought in 120,000 Germans to settle on their lands and had also taken hostages. Caesar would be defending the Gauls against ongoing German migrations.
Caesar agreed to speak with Ariovistus. Like the Aedui, who were among the complaining tribes, Arivistus' Suebi were an official friend of the Romans.
Ariovistus refused Caesar's request for a meeting. He said Caesar should come to him and wondered why Rome was interfering.
Caesar's response was to tell him not to bring Germans into Gaul and to restore hostages taken from the Aedui. Refusal would bring reprisals.
Ariovistus had had similar military success to Caesar and saw no reason to obey.
Caesar says that the Aedui claimed the Germans had made additional raids and the Treveri Gauls said the Suebi were preparing to send huge numbers of Germans (100 clans) across the Rhine into Gaul; thereby violating all the terms Caesar had issued.
Caesar set out with his army and learned the Germans were advancing on the town of Vesontio, the tribal center of the Gallic Sequani who had been allies of the Suebi. Caesar didn't want the food supplies there to fall into German hands so he raced his troops there and then let them rest. Caesar's troops were worried by stories they'd heard of the Germans and fearful that they would run short of grain as had happened in the earlier battles. Caesar cajoled the legions and made some compromises, but they set out towards Ariovistus, now 24 miles away.
Ariovistus now agreed to a talk.
At their meeting halfway between their camps, both leaders maintained their earlier positions. Ariovistus considered the area they were in his province, just as Caesar was proconsul with jurisdiction over the Transalpine province. He offered his support to Caesar if he withdrew. A fight broke out among their associates and the meeting broke up. A new one was arranged a few days hence and when Caesar sent two envoys, the Germans arrested them.
Ariovistus advanced his troops to about 6 miles from Caesar's camp and stationed them on high ground. The next day he led them further and cut off Caesar from the supply lines. Still there was nothing more than minor skirmishes.
Caesar decided to reopen his supply lines. He led out his army in 3 lines with the 3d line to construct a new camp for 2 legions while the other 2 were available for fighting. when they finished, the two legions were in position to protect delivery of supplies from the allies.
From prisoners taken in small fights, the Romans learned the Germans were waiting for the full moon.
The next day Caesar led his troops against the Germans. The Germans responded with vigor, but Roman tactics soon led to panic among the Germans. As they fled, Caesar led the pursuit. Ariovistus managed to escape.
