As Ariovistus made incursions against the Roman allied Gallic Celts, Caesar sought initially to resolve the matter diplomatically, at least according to his writings. Ariovistus, the Suebi chief, had already conquered much of eastern Gallic territory only a few years prior, but Caesar was able to reverse the situation through diplomacy. Ariovistus was named a friend and ally of the Roman people, but this relationship proved to be short lived. Caesar's initial attempts in 58 BC to negotiate with the Germanic were defiantly rebuffed. Ariovistus it seems, viewed this part of Gaul as his territory, and Caesar's presence was considered a violation of their treaty. Caesar quickly recruited additional forces and prepared for the impending conflict. Additional missions sent to end the stand off diplomatically were failures.
Caesar estimates that over 120,000 Germanic warriors had crossed the Rhine into Gaul at this point, occupying the land of the Sequani and harassing the Aedui, as well as other tribes. Caesar received word that additional warriors were crossing the Rhine to siege the Sequani town of Vesontio, so he made haste to garrison the town. By the time the enemy arrived, the Romans were safely entrenched and Ariovistus decided to open negotiations. Caesar and the Germanic chief met face to face to discuss the situation, but little was accomplished. Ariovistus argued that he had as much right as the Romans to conquer and control in order to prevent additional warfare. Caesar while acknowledging this very simple and parallel Roman principal suggested that Ariovistus' rights were null and void due to his failure to help against the Helvetii. It was quite clear that neither side was going to back down from the other and war was a foregone conclusion. Caesar, however, encountered new problems. His legions, now in such proximity to the wild Germans were suffering from terrible moral. Any battle to come would surely result in panic from his men in the face of the enemy's ferocity. Caesar delivered a rousing speech specifically praising his famous 10th legion and suggesting that if the others were too frightened, he would go into battle with only this one. Being called out by the beloved commander, as he would do often to quell trouble in the future, settled the men and they were thereafter eager to join the coming fight.
Somewhere near modern Besancon, with all diplomacy exhausted, the Romans and the Germanics finally met in battle, sometime in September of 58 BC. Impatient at delaying tactics employed by Ariovistus, Caesar launched a full assault on the enemy camp. He himself accompanied his right wing which seemingly would need the most support. The battle that followed was an epic struggle between two well matched armies. Caesar wrote that his men fought valiantly sweeping the Suevi on the left flank, "There were found very many of our soldiers who leaped upon the phalanx, and with their hands tore away the shields, and wounded the enemy from above. Although the army of the enemy was routed on the left wing and put to flight, they pressed heavily on our men from the right wing, by the great number of their troops." Seeing the trouble on the right, Caesar's young lieutenant Publius Licinius Crassus, son of the triumvir, led a cavalry charge that turned the tide in Roman favor. The Suevi were routed on the field, with estimates as high as 25,000 being killed. The survivors rushed back to the Rhine to cross back home including Ariovistus who escaped in a small boat.
The surviving Suebi then had to deal with another Germanic tribe, the Ubii, who harassed and attacked them as they returned home. The Suebi survived to fight another day however, and Caesar would be forced to face them again just a few years later. Unfortunately, though, never mentions the fate of Ariovistus, but having concluded two very important wars in one campaign season, Caesar put his army into winter quarters among the Sequani. He appointed Labienus in command, and set out in person for Cisalpine Gaul to attend to political business.