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Campaign against the Suebi

Caesar then turned his attention to the Germanic Suebi, whom he also wished to conquer. The Senate had declared Ariovistus, king of the Suebi, a "friend and ally of the Roman people" in 59 BC, so Caesar needed a convincing casus belli to betray the Suebi. He found his excuse following victory over the Helvetii. A group of Gallic tribes congratulated him and sought to meet in a general assembly, hoping to leverage the Romans against other Gauls. 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. 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. With the attack of the Harudes (an apparent Suebi ally) 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 BC.

Learning that Ariovistus intended to seize Vesontio, the largest Sequani town, Caesar marched towards it and arrived before Ariovistus. Ariovistus sent emissaries to Caesar requesting a meeting. They met under a truce at a knoll outside of town. The truce was violated when Germanic horsemen edged towards the knoll and threw stones at Caesar's mounted escort. Two days later, Ariovistus requested another meeting. Hesitant to send senior officials, Caesar dispatched Valerius Procillus, his trusted friend, and Caius Mettius, a merchant who had traded successfully with Ariovistus. Insulted, Ariovistus threw the envoys in chains. Ariovistus marched for two days and made camp two miles (3.2 km) miles behind Caesar, thus cutting off his communication and supply lines with the allied tribes. Unable to entice Ariovistus into battle, Caesar ordered a second smaller camp built near Ariovistus' position.

The next morning Caesar assembled his allied troops in front of the second camp and advanced his legions in towards Ariovistus. Each of Caesar's five legates and his quaestor were given command of a legion. Caesar lined up on the right flank. Ariovistus countered by lining up his seven tribal formations. Caesar was victorious in the ensuing battle due in large part to the charge made by Publius Crassus, son of Marcus Crassus. As the Germanic tribesmen began to drive back the Roman left flank, Crassus led his cavalry in a charge to restore balance and ordered up the cohorts of the third line. As a result, the whole Germanic line broke and began to flee. Caesar claims that most of Ariovistus' one-hundred and twenty thousand men were killed. He and what remained of his troops escaped and crossed the Rhine, never to engage Rome in battle again. The Suebi camping near the Rhine returned home. Caesar was victorious. In one year he had defeated two of Rome's most feared enemies. After this busy campaigning season, he returned to Transalpine Gaul to deal with the non-military aspects of his governorship. At this point it is possible he had already decided he would conquer all of Gaul.