The battle of Arausio (6 October 105 BC) was the most serious Roman defeat during the Cimbric Wars and saw the defeat and destruction of two Roman armies, apparently leaving Rome open to attack.
The Cimbri first appeared in the records in 113 BC, when they defeated a Roman army near Noreia, to the north-east of the Alps, after migrating south from Jutland. They then disappeared into Gaul, before reappearing in 109 or 108 BC, when they defeated the consul Marcus Junius Silunus, probably somewhere on the borders of the Roman province in Gaul.
The battle of Arausio isn't terribly well documented, but for once we do have a fixed date. Granius Licinianus places it on the day before the nones of October. The nones of October fell on the 7th, placing the battle of 6 October 105 BC.
Our sources disagree on which tribes were present. Orosius and Eutropius list the Cimbri, Teutones, Tigurini and Ambrones. The periochae of Livy, Granius Licinianus and Cassius Dio just mentions the Cimbri. Plutarch gives the Cimbri and Teutones in his life of Sertorius. The modern consensus is that this battle only involved the Cimbri, and the other tribes became involved in the fighting in 102-101 BC, but it isn't possible to be entirely sure about that. We also have little or no information about the events leading up to the campaign, and our sources all begin with the Roman forces in place.
The Romans had two armies in southern Gaul in 105 BC. The first was commanded by the proconsul Q. Servilius Caepio, who had been given Roman Gaul as his province in the previous year. The second was commanded by Cn. Mallius Maximus, one of the consuls for 105 BC. Officially Mallius outranked Caepio, but Caepio resented being superceded and refused to cooperate with his colleague. The two men agreed to split the province in two, with the Rhone being the boundary, but our sources don't say which of the Roman commanders got which side of the river.
There were probably at least two battles in the campaign. The first is mentioned in the Periochae of Livy, Granius Licinianus and Dio Cassius. The former consul Marcus Aurelius Scaurus, who was serving under Mallius, suffered a defeat while commanding an independent army and was captured by the Cimbri. Livy and Granius Licinianus both report that Scaurus was summoned before a council of the Cimbri, where he attempted to convince them not to invade Italy. Granius Licinianus also says that he was offered command of the Cimbric army, but refused the role. He was then killed, possibly by a young chieftain called Boiorix.
Mallius was worried by this setback, and asked Caepio to join with him, so their armies could fight united. Caepio refused to cooperate, but he did cross the Rhone into Mallius's province, where he camped between Mallius and the Cimbri, possibly in an attempt to make sure that he would get the glory of any victory. The two camps were near to Arausio (modern Orange).
The Cimbri didn't realise that the Romans were suffering from such serious internal disagreements. They saw two full Roman armies, camping close together, and decided to send envoys to discuss peace. The Cimbri sent their ambassadors to Mallius, the senior man, but this so angered Caepio that he almost attacked them.
The Romans really needed to cooperate, for they were probably very badly outnumbered. Plutarch reports that the Cimbri had 300,000 fighting men. We don't know exactly how big the Roman army was, but they lost 70,000-80,000 men and the army was almost wiped out, suggesting that it wasn't much bigger than that. If those figures are at all accurate then the Romans were outnumbered by three to one.
According to Granius Licinianus the Cimbri attacked on the day after the meeting. We have no details of the battle itself, but the Romans were defeating, losing both of their camps. Granius Licinianus gives the Roman casualties as 80,000 regular and light troops killed, and gives his source as Rutilius Rufus, one of Marius's officers. Livy and Orosius give figures of 80,000 soldiers and 40,000 servants and camp followers killed. Amongst the dead were two of Mallius's sons. Amongst the survivors was the young Sertorius, who managed to escape by swimming across the Rhone. This makes it one of the most costly defeats in Rome history, worse even than the famous disaster at Cannae.
The defeat at Arausio had a dramatic impact in Rome. The other consul, Publius Rutilius Rufus, took command, and raised a new army in Italy, using gladiators to train the new recruits. The disaster also played a major part in the rise of Marius, then serving as the proconsul in command of the Jugurthine War. News of the disaster in Gaul was followed by news of the capture of Jugurtha and the end of that war, after seven years of frustration. Marius was elected as consul for a second time in absentia, technically breaking Roman election law, but with massive popular and senatorial support. This was the first of five consecutive terms, making Marius the dominant man in the state for the rest of the war.
The two defeated commanders were disgraced. Caepio was put on trial, deprived of his imperium, and imprisoned. He may have died in prison, but it is more likely that he was freed but exiled, ending his life at Smyrna, becoming a citizen of that city. Mallius was also exiled.
The Cimbri didn't invade Italy in 105 BC, but instead turned west and invaded Spain, where Roman authority appears to have been in decline for some time. This campaign lasted until 103 BC, but eventually they were forced back into Gaul. This gave Marius time to train his new army, and he was able to win the crucial final battles of the war, at Aquae Sextiae (102 BC) and Vercellae (30 July 101 BC)