In 61 BCE, Ariovistus, chieftain of the Suebi tribe and a king from the Germanic peoples, resumed the tribe's migration from eastern Germania to the Marne and Rhine regions. Despite this migration encroaching on Sequani land, they sought Ariovistus' allegiance against the Aedui. In 61 BCE, the Sequani rewarded Ariovistus with land following his victory at the Battle of Magetobriga. Ariovistus settled the land with 120,000 of his people. When 24,000 Harudes joined his cause, he demanded the Sequani give him more land to accommodate them. This demand concerned Rome because if the Sequani conceded, Ariovistus would be able to take all of their land and attack the rest of Gaul.

Following Caesar’s victory over the Helvetii, most of the Gallic tribes congratulated him and sought to meet in a general assembly. Diviciacus, the head of the Aeduan government and spokesmen for the Gallic delegation, expressed concern over Ariovistus' conquests and for the hostages he had taken. Diviciacus demanded that Caesar defeat Ariovistus and remove the threat of a Germanic invasion otherwise they would have to seek refuge in a new land. Not only did Caesar have a responsibility to protect the longstanding allegiance of the Aedui, but this proposition presented an opportunity to expand Rome's borders, strengthen loyalty within Caesar’s army and establish him as the commander of Rome’s troops abroad.

The senate had declared Ariovistus a "king and friend of the Roman people" in 59 BCE, so Caesar could not easily declare war on the Suebi tribe. Caesar said he could not ignore the pain the Aedui had suffered and delivered an ultimatum to Ariovistus demanding that no Germanic tribesman cross the Rhine, the return of Aedui hostages and the protection of the Aedui and other friends of Rome. Although Ariovistus assured Caesar that the Aedui hostages would be safe as long as they continued their yearly tribute, he took the position that he and the Romans were both conquerors and that Rome had no jurisdiction over his actions. With the attack of the Harudes on the Aedui and the report that a hundred clans of Suebi were trying to cross the Rhine into Gaul, Caesar had the justification he needed to wage war against Ariovistus in 58 BCE.