The Battle of Bibracte was fought between the Helvetii

Helvetii

The Helvetii were a Celtic tribe or tribal confederation occupying most of the Swiss plateau at the time of their contact with the Roman Republic in the 1st century BC...

 and six Roman legions, under the command of Gaius Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

. It was the second major battle of the Gallic Wars

Gallic Wars

The Gallic Wars were a series of military campaigns waged by the Roman proconsul Julius Caesar against several Gallic tribes. They lasted from 58 BC to 51 BC. The Gallic Wars culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the...

.

After following the migration of the Helvetii and defeating them, Caesar, around 20 June, moved towards Bibracte

Bibracte

Bibracte, a Gaulish oppidum or fortified city, was the capital of the Aedui and one of the most important hillforts in Gaul. It was situated near modern Autun in Burgundy, France. The material culture of the Aedui corresponded to the Late Iron Age La Tène culture,In 58 BC, at the Battle of...

 (approximately 18 miles away from their camp) to obtain the supplies promised by his allies, the Aedui

Aedui

Aedui, Haedui or Hedui , were a Gallic people of Gallia Lugdunensis, who inhabited the country between the Arar and Liger , in today's France. Their territory thus included the greater part of the modern departments of Saône-et-Loire, Côte-d'Or and Nièvre.-Geography:The country of the Aedui is...

. Informed by deserters of Lucius Æmilius, commander of the cavalry, the Helvetii took this occasion to turn and harass Caesar's rear guard. When Caesar observed this, he sent his cavalry to delay the attack. He placed the Seventh (Legio VII Claudia

Legio VII Claudia

Legio septima Claudia Pia Fidelis was a Roman legion. Its emblem, like that of all Caesar's legions, was the bull, together with the lion....

), Eighth (Legio VIII Augusta

Legio VIII Augusta

Legio octava Augusta was a Roman legion created by Pompey in 65 BC, along with the 6th, 7th & 9th, and continuing in service to Rome for at least 400 years thereafter....

), Ninth (Legio IX Hispana

Legio IX Hispana

Legio Nona Hispana was a Roman legion, which operated from the first century BCE until mid 2nd century CE. The Spanish Legion's disappearance has raised speculations over its fate, largely of its alleged destruction in Scotland in about 117 CE, though some scholars believe it was destroyed in the...

), and Tenth legions (Legio X Equestris

Legio X Equestris

Legio X Equestris , a Roman legion, was levied by Julius Caesar in 61 BC when he was the Governor of Hispania Ulterior. The Tenth was the first legion levied personally by Caesar, and was consistently his most trusted...

), organized in Roman fashion (triplex acies), at the foot of a nearby hill, the top of which he occupied himself, along with the Eleventh (Legio XI Claudia

Legio XI Claudia

Legio undecima Claudia was a Roman legion. XI Claudia dates back to the two legions recruited by Julius Caesar to invade Gallia in 58 BC, and it existed at least until early 5th century, guarding lower Danube in Durostorum...

) and Twelfth (Legio XII Fulminata

Legio XII Fulminata

Legio duodecima Fulminata , also known as Paterna, Victrix, Antiqua, Certa Constans, and Galliena, was a Roman legion, levied by Julius Caesar in 58 BC and which accompanied him during the Gallic wars until 49 BC. The unit was still guarding the Euphrates River crossing near Melitene at the...

) Legions and all his auxiliaries. The baggage train was also assembled near the summit, where it could be guarded by the forces present there.

After having driven off Caesar's cavalry and with their own baggage train secured, the Helvetii engaged "In the seventh hour", approximately noon or one o'clock. According to Caesar, his hill-top battle line easily threw back the Helvetii onslaught by using pila

Pilum

The pilum was a javelin commonly used by the Roman army in ancient times. It was generally about two metres long overall, consisting of an iron shank about 7 mm in diameter and 60 cm long with pyramidal head...

 (javelins) at about 10 to 15 yards. The legions then counterattacked, driving the Helvetii approximately a thousand paces back to the hill where their Helvetic baggage train sat.

While the legions pursued the Helvetii across the plain in between the hills, the Boii

Boii

The Boii were one of the most prominent ancient Celtic tribes of the later Iron Age, attested at various times in Cisalpine Gaul , Pannonia , in and around Bohemia, and Transalpine Gaul...

 and Tulingi

Tulingi

The Tulingi were a small tribe closely allied to the Celtic Helvetii in the time of Julius Caesar's conquest of Gaul. Their location is unknown; their language and descent are uncertain. From their close cooperation with the Helvetii it can be deduced that they were probably neighbours of the latter...

 arrived with fifteen thousand men to assist the Helvetii, flanking the Romans on one side. At that point the Helvetii returned to the battle in earnest. When the Tulingi and the Boii started circumventing the Romans, Caesar regrouped his third line to resist the assault of the Boii and Tuligni, keeping his primary and secondary committed to chasing the Helvetii.

The battle lasted many hours into the night until the Romans finally took the Helvetic baggage train, capturing both a daughter and son of Orgetorix

Orgetorix

Orgetorix was a wealthy aristocrat among the Helvetii, a Celtic-speaking people residing in what is now Switzerland during the consulship of Julius Caesar of the Roman Republic. In 61 BC he convinced the Helvetians to attempt to migrate from Helvetian territory to south-western Gaul...

. According to Caesar, 130,000 enemy personnel escaped into the night. Unable to pursue on account of battle wounds and the time it took to bury the dead, Caesar rested three days before he followed the fleeing Helvetii. These, in turn, had managed to reach the territory of the Lingones

Lingones

Lingones were a Celtic tribe that originally lived in Gaul in the area of the headwaters of the Seine and Marne rivers. Some of the Lingones migrated across the Alps and settled near the mouth of the Po River in Cisalpine Gaul of northern Italy around 400 BCE. These Lingones were part of a wave of...

within four days of the battle. Caesar warned the Lingones to not assist them, prompting the Helvetii and their allies to finally surrender.

Caesar claimed that 130,000 of the Helvetii and her allies escaped yet only 110,000 returned home.

Also according to Caesar the census totals of the tribes at the start of the war were:

Tribe	Population census
Helvetii	263,000
Tulingi	36,000
Latobrigi	14,000
Rauraci	23,000
Boii	32,000
Total	368,000
Combatants	92,000