Ariovistus (fl.61-58 B.C.) was a Suebian chief who led a large force of Germans across the Rhine in the years just before the outbreak of Caesar's Gallic War. He carved out a sizable kingdom in Alsace before being defeated by Caesar and forced to retreat back across the Rhine and into obscurity.
Ariovistus was invited across the Rhine by the Sequani, who were involved in a long running dispute with their neighbours to the west, the Aedui, at least in part over who had the right to charge tolls on the Saone. It isn't entirely clear when he crossed the Rhine. He won a major battle against the Aedui at Admagetobriga in 61 B.C., when according to Caesar he took advantage of the exhaustion of the Gauls after a long war. There had also been time for Ariovistus's original 15,000 men to swell to a force of 120,000 by 58 B.C., but Caesar's commentary can also be read to suggest that the Germans had only recently crossed the Rhine in 61 B.C.
This might explain why the Romans failed to support their allies, the Aedui, as the war turned against them. After Ariovistus's crushing victory at Admagetobriga the Druid Diviciacus was sent to Rome to request aid, but all he was able to secure was a Senate decree ordering the governor of Transalpine Gaul to protect the interests of the Aedui (late 61 B.C.). This coincided with the Allobrogian revolt within the Roman Province, and so no help was offered. In 59 B.C. the Romans even gave Ariovistus the title of Friend of the Roman People, probably in an attempt to buy him off.
Ariovistus's success probably triggered the series of events that led to his defeat. Having won their war he began to make increasingly harsh demands of the Sequani, who were forced to surrender first one third and then two thirds of their territory. Other Germans also crossed the Rhine and began to raid into Gallic territory. Most significantly the Helvetii tribe of modern Switzerland decided to leave their current home and migrate west across Gaul to seek conquests on the west coast. This migration inevitably dragged in the Romans, who always wanted stability just beyond their borders. In 58 B.C. Caesar defeated the Helvetii in two battles, and forced them to return to their homeland.
Caesar now had an army of six legions camped outside the Roman province of Transalpine Gaul. Ariovistus was his obvious next target. The Romans had had a dread of German hordes ever since the invasion of the Cimbri and the Teutones in 113-101 B.C., so the presence of an increasingly powerful German king west of the Rhine was an obvious concern. Ariovistus was also dangerously close to the rebellious Allobroges in the Roman Province. It is clear that he was also throwing his weight around in Gaul, demanding hostages and tribute from neighbouring tribes, and a delegation of Gallic leaders went to Caesar to request Roman aid.
Ariovistus now found himself facing a Roman army six legions strong. Caesar advanced towards the Sequani capital of Vesontio (modern Besançon), and soon captured the city. The fall of Vesontio convinced Ariovistus that he should meet with Caesar. A conference took place on mound of earth located between the camps of the two armies. According to Caesar's account of the battle Ariovistus's main theme was that he had crossed the Rhine at the invitation of the Gauls, and that the Romans were invading his legitimate Gallic Province. Neither men was willing to make any concessions, and the meeting soon broke up.
Over the next few days the two armies manoeuvred around each other. According to Caesar Ariovistus was waiting for the new moon, to satisfy the results of a divination that said he could not win before that. Caesar decided to force a battle before the new moon, and on the day after learning of the divination he advanced towards the German camp. Ariovistus was forced to fight, and as predicted suffered a heavy defeat (battle of Vesontio).
The German retreat continued for fifty miles, until the survivors reached safety across the Rhine. Ariovistus was one of survivors, but both of his wives and one of his daughters were killed in the retreat and his surviving daughter was captured. Ariovistus's fate after this is unknown.