The Cimbric War (113-101 BC) saw the Romans suffer a series of serious defeats at the hands of the Cimbri, Teutons and other tribes, before the consul Marius won a series of victories that ended the threat to Italy.
The Cimbric War was an unusual conflict, in that Rome's main opponents appeared and disappeared at irregular periods. The Romans suffered a series of crushing defeats on the north-western frontiers of their empire, which inevitably caused a series of panics in Rome, but until the final campaigns of 104-101 BC the threat to Italy never actually materialised, and the victorious tribes either moved on elsewhere or disappeared completely from our sources.
We also know very little about Rome's main opponents in this war. The first tribe to appear, the Cimbri, only appears in the sources in 113 BC, at the start of the war. We know very little about them - they might have been Germanic or Celtic, and probably lived in Jutland before the start of their migration. The standard reason given for the migration in the ancient sources was rising sea levels, which may well have been the case if they were living on the Jutland coast of the North Sea.
The second major tribe, the Teutones, probably entered the war towards its final stages (although Appian and Velleius both have them involved from the start). They were recording as living on the North Sea coast in the 320s BC, and again may have been Celtic or Germanic.
The Ambrones were a smaller tribe, possibly a Gallic group from the modern Low Countries, also forced to move by rising sea levels.
These northern tribes also found at least one southern ally, the Tigurini, from the area of modern Switzerland, who took part in the final part of the war.
Rome only gained a permanent present in southern Gaul a decade before the appearance of the Cimbri. Before then there had been no direct Roman control land route between Italy and Spain. They had allies in the area, in particular the Greek city of Massilia (Marseille), and from time to time Roman armies campaigned in the area to defend their allies. This began to change after the campaigns of C. Sextius Calvinus, one of the consuls for 124 BC, who defeated the Salluvii, Ligures and Vocontii, and established a new Roman settlement at Aquae Sextiae (123 BC). This provoked a reaction from King Bituitus of the Arverni, who claimed to be the overking of the area. He suffered two defeats, the first at the hands of Cn Domitius Ahenobarbus, consul of 122 BC and the second against Q. Fabius Maximus, consul of 121 BC (in August 121). In the aftermath of these victories Domitius created a new Roman province, Transalpine Gaul, stretching from the Pyrenees to Geneva. Some of the neighbouring tribes became Friends and Allies of the Roman People, providing something of a buffer for the new Province, but also giving the Romans a justification for later interference in Gaul, and dragging them into a series of often costly conflicts. The Romans also established another colony, at Narbo, in 118 BC, and at the same time began to construct a road connecting Italy to Spain, the Via Domitia.
The first sign of the upcoming crisis was a request for help from the Taurisci, new Roman allies in the north-eastern Alps. They reported that a large northern tribe, the Cimbri, was migrating towards them, and requested help. The Romans sent one of the consuls for 113 BC, Cn. Papirius Carbo, to investigate the new threat. At first Carbo simply attempted to defend the line of the Alps, but he then advanced towards the Cimbri, and after entering into negotiations with them attempted to ambush them. He was defeated near Noreia, and his army was only saved from total destruction by nightfall and a thunderstorm. After this victory, the Cimbri moved into Gaul, where they disappeared from the records.
The Cimbri reappeared in 109 BC, probably on the border between Italy and Gaul, although the details of this clash are so vague that it isn't possible to be sure. They defeated an army under the consul Marcus Junius Silunus, probably in 109 BC, and asked the Romans for land in return for military service. This demand was turned down, and the Cimbri then disappear from the sources once again.
This didn’t end the problems for the Romans. In 107 BC the Tigurini decided to raid southern Gaul. One of the consuls for the year, Cassius Longinus, was sent to deal with the Tigurini, but somewhere in the south-west of Gaul was ambushed, defeated and killed. The surviving members of his army had to surrender hostages and half of their possessions before they were allowed to leave in safety.
The main recorded event of 106 BC was the siege of Tolosa (Toulouse) by the consul Q. Servilius Caepio. The Romans were let into the city, where they found a famous treasure. The 'treasure of Tolosa' disappeared on the way to Rome, leading to a scandal that was remembered for many years afterwards. Pompeius Trogus even suggested that the defeat at Arausio was punishment for the theft of the treasure.
The Cimbri reappeared in 105 BC. Caepio was already in southern Gaul, and the Romans sent another army, under the consul Cn. Mallius Maximus, giving them around 80,000 men. However the two commanders were unable to cooperate, with Caepio getting the blame in the ancient sources. The two Roman armies camped a short distance apart and didn't cooperate, allowing the Cimbri to inflict a crushing defeat on them, almost destroying both armies (battle of Arausio, 7 October 105 BC), one of the worst defeats ever suffered by the Romans.
In the aftermath of this victory, the Cimbri once again turned away from Italy. This time they decided to invade Spain, where they devastated large areas before being defeated by the Celtiberians and forced back to Gaul. This is where Livy has them join up with the Teutons, in the lands of the Veliocassians, which would place the meeting somewhere in the north of France, on the lower Seine.
In Rome the response was to raise a new army, and at the same time to elect Marius as one of the consuls for 104 BC. He had just captured Jugurtha, ending the long Jugurthine War, and his election appears to have been popular at all levels. However Marius was faced with an unexpected problem, when the Cimbri failed to appear. He had to both find something for his men to do, and make sure that he was still in power when the invasion did actually take place. We don't have much evidence of what he did with his army in 104-103 BC. We know that he got his men to dig a canal linking the Rhone to the Mediterranean, to avoid the muddy river mouth. Plutarch's life of Sulla suggests that Marius also used the time to re-establish Roman power in southern Gaul and the Alps.
The Cimbri finally returned from Spain in 103 BC, and formed an alliance with the Teutones, Ambrones, Tigurini and Toygeni. The new allies agreed to invade Italy in 102 BC, advancing along two routes. The Teutones and Ambrones would attack from the north-west, the Cimbri and other tribes from the north-east.
The Teutones and Ambrones moved first, advancing down the Rhone Valley. Marius rushed from Italy to join the army the Rhone, but he then refused to allow his troops to attack the tribes on the Rhone. Instead he stayed in his camp, allowing his men to get used to their opponents, and the tribes to wear themselves down attacking his camp. After a few days the tribes moved off, heading for Italy. Marius followed close behind them, until eventually he reached his chosen battlefield at Aquae Sextiae (modern Aix-en-Provence). There he was able to defeat the Ambrones in a preliminary battle near a river, and convince the Teutones to attack uphill towards his camp. Both tribes were totally defeated in the battle, and the threat from the north-west was eliminated.
Unfortunately Marius's co-consul, Q. Lutatius Catulus, was less effective. He was unable to defend the line of the Alps, and was then forced to abandon the Adige, giving the Cimbri possession of the north-east of Italy. The Cimbri then paused, possibly to recover from the mountain passage or to wait for the Teutones and Ambrones. This gave Marius time to visit Rome and then summon his army from Gaul. The combined Roman armies then inflicted a second heavy defeat on the invaders at Vercellae or the Raudian Plain (30 July 101 BC). The Cimbric army was totally crushed in this battle, ending the threat to Rome.
The final army, that of the Tigurini, never crossed the Alps. They were still waiting in Noricum, in the the north-eastern Alps, when news reached them of the defeat of their allies, and they dispersed without fighting.
The Cimbric War had kept the Romans in fear for over a decade, between their first unexpected appearance to the north-east of the Alps to the final invasion of Italy in 102-101 BC. The Romans were always sensitive to any threat from the north, having long memories of the Gallic sack of Rome in 390 BC. The Cimbri in particular played into this fear, appearing and disappearing seemingly at random, appearing to be an unstable vast horde that outnumbered any Roman army capable of being raised, and defeating a series of increasingly large Roman armies. Marius's eventual victory ended the threat, but it also laid some of the seeds for the series of civil wars that would threaten the future of the Roman Republic in the decades to follow.