The battle of the Arar (June 58 BC) was the first significant victory won by Julius Caesar, and marked the unusually late start of his military career. During his year as Consul (59 BC) Caesar had secured Cisalpine Gaul and Illyricum as his proconsular provinces, and even after Transalpine Gaul was added to his responsibilities towards the end of the year his original provinces seemed most likely to provide him with a chance for the military glory that was an essential part of a successful political career in Rome. When Caesar took over his provinces three of his four legions were based at Aquileia, in the north-east of Italy, to guard against a potential threat from the Dacians. Transalpine Gaul (known as 'The Province') to the Romans was guarded by a single legion.
The Romans were aware that the Helvetian tribe had been planning to migrate from its home in modern Switzerland, where they were threatened by an increasing German presence west of the Rhine, to the west coast of Gaul where they intended to conquer a new home. This military migration threatened the stability of southern Gaul, and the creation of a strong Helvetian state between the Province and Roman Spain posed a real threat to Rome, but the death of Orgetorix, the Helvetian nobleman largely responsible for the migration, seems to have convinced the Romans than the move wouldn't take place.
The Romans were wrong. In the spring of 58 BC the Helvetii burnt their villages, destroyed anything they couldn’t take with them, and on 28 March the Helvetii, Raurici, Tulingi, Latobrigi and Boii tribes began to move west. At least 300,000 people were involved in this mass-migration, and one quarter of them were capable of fighting.
The Helvetii's final preparations alerted Caesar, and in mid-March he was able to leave the vicinity of Rome, arriving at Geneva just before the Helvetii. With only one Legion at his disposal Caesar was forced to play for time. When the Helvetii asked for permission to cross the Province, Caesar pretended to consider the request, and used the time he gained to build a line of fortifications on the southern banks of the Rhone and to destroy the bridge over the river at Geneva.
In mid-April Caesar informed the Helvetii that they would not be allowed to cross the Roman Province. The Helvetii were forced to seek an alternative route through the lands of the Aedui, a tribe allied to Rome. Dumnorix of the Aedui, an important member of the tribe, convinced the Sequani to allow the migration to pass through their territory. The Helvetii would then have to cross the Saone River and move through Aedui territory on their way to the west.
Although the immediate threat to the Province was gone Caesar was still determined to block the migration. He dashed back to Italy where he collected the three veteran legions from Aquileia and two new legions that had been raised in Cisalpine Gaul. He then returned back across the Alps and united his six legions just to the north of the Rhone.
By this time the Helvetii had reached the Saone (known to the Romans as the Arar) and were preparing to cross the river into Aedui territory. According to Caesar fighting had already broken out between the Helvetii and the Aedui, suggesting that Dumnorix had fallen out of favour and the pro-Roman faction was back in power. The Aedui asked their Roman allies for assistance, and Caesar was more than happy to respond (his own account suggests that Caesar had originally planned to let the Helvetii reach the west coast before intervening).
Caesar was now informed that the Helvetii had already begun to cross the Saone. The tribe was divided into four cantons, and three of the four were already on the west bank, but the members of the Tigurine canton were still on the east bank. Caesar realised that this was too good a chance to miss, and during the third watch of the night (some time after midnight) he led three of his legions out of their camp and made a night march towards the river crossing.
The Romans surprised the Tigurini while they were preparing to cross the river. 'A great part of them' were cut down, and the rest escaped into some nearby woods. Caesar's account of the battle doesn't include any casualty figures, perhaps suggesting that a sizable number of the Tigurini escaped.
In the aftermath of the battle Caesar threw a bridge across the Saone, and his army crossed the river in a single day. This impressed the Helvetii enough for them to open negotiations, but these quickly failed. The Helvetii resumed their migration with the Romans following close behind. After fifteen days of this pursuit the Romans were running short of supplies, and Caesar decided to make a diversion to Bibracte, where he hoped to find fresh supplies. The Helvetii decided to take this chance to attack the Romans, but instead suffered a decisive defeat at Bibracte.