The battle of Aquae Sextiae (102 BC) was the first of Marius's great victories during the Cimbric War and saw him destroy the Teutones and the Ambrones, two of the smaller tribes involved in the great invasion of Italy.
In 105 BC the Cimbri had crushed a Roman army at Arausio (6 October 105 BC), apparently leaving Italy open to invasion, but they chose instead to invade Spain, where they spent most of 105-103 BC. News of the disaster at Arausio reached Rome just before the news that Marius had captured the Numidian king Jugurtha, bringing to an end the lengthy Jugurthine War. Unsurprisingly Marius was the overwhelming choice as one of the consuls of 104 BC, despite having already served as consul in 107 BC, well within the ten year limit for a second term.
The absence of the Cimbri gave Marius plenty of time to train his army, which was built around a core of troops recruited in Italy in the aftermath of Arausio. However it also meant that he had to arrange for his re-election in 103 BC, for an almost unprecedented second consecutive term. Rumours that the tribes were planning an invasion in 103 helped him get reelected in that year, and popular agitation, led by one of Marius's supporters, the tribune of the plebs L. Appuleius Saturninus, secured him the election for 102 BC.
The Cimbri finally reappeared in 102 BC, but this time they were at the head of a much larger tribal coalition, which included the Teutones (another tribe from the North Sea coastline), the Ambrones and two Alpine tribes, the Tigurini and the Toygeni. After their earlier victories the Cimbri had decided not to invade Italy, but they now decided to attempt a two pronged invasion. The Teutones and the Ambrones were to invade from Gaul into the north-west of Italy, the Cimbri and the Tigurini from the north-east.
As is so often the case for this war, the exact details of the battle are unclear. At least some of the tribes advanced down the Rhone, towards Marius's camp at the junction of the Isere and Rhone Rivers (close to modern Valence). According to Orosius, all of the tribes were together at this point, and only split up after they failed to persuade Marius to come of out his camp to fight. According to Plutarch only the Teutones and the Ambrones came down the Rhone, while the Cimbri marched across country towards Noricum. This version of events makes rather more sense.
Marius was determined only to fight on ground of his own chosing, and used this opportunity to get his men used to the sight of the tribes, to reduce the mystery associated with them. After three days of failed attacks on his camp, the Teutones and Ambrones decided to begin their invasion of Italy. According to Plutarch it took six days for the barbarian army to pass Marius's camp. Once they were past, Marius broke camp and followed close behind, always making sure that he camped in strongly fortified positions, to deny the tribes any chance to attack.
This continued until the two armies were in the vicinity of Aquae Sextiae (modern Aix-en-Provence). The barbarians were now getting dangerously close to Italy, and so Marius decided to fight (possibly having picked out this location in the previous two years). He chose to camp on a hill overlooking a river, protected on both flanks by woods and ravines. However the hilltop lacked water. Marius ordered his soldiers to fortify their camp before he would allow them to go for warter, but the camp followers were less patient. They went down to the river, where they became involved in a clash with the Ambrones (said to be 30,000 strong). This soon expanded into a major battle, with more and more of Marius's men getting involved, starting with the Ligurians. The Ambrones were caught against the river, and suffered a very heavy defeat.
Several days then passed without any more fighting. Marius refused to come down from the hill, but instead planned to force the Teutones to come up to attack him. He posted 3,000 legionaries in the woods on one flank, where they were hidden from the Teutones. Four days after the clash at the river he was ready to fight. He moved his infantry out of their camp, and formed up at the top of the hill. The cavalry were sent down to the plains to harass the Teutones, but the rest of his army was ordered to stay at the top.
The Teutones fell for the trap, and advanced up the hill. This probably forced them into a narrow area between the ravines and trees, reducing the impact of their superior numbers. They were also forced to fight after a climb, and were slowly forced back down the hill by Marius's men. When they reached the base, Marecellus attacked their rear, throwing the entire force into confusion. The Teutones soon broke and fled, suffering very heavily in the resulting pursuit. Plutarch says that they lost 100,000 men killed or captured. Orosius gives higher figures of 200,000 killed, 8,000 captured and 3,000 who escaped. The leaders escaped for a short time, but were captured in the Alps.
In the aftermath of this victory, news arrived that Marius had been elected consul for a fifth time. Worse news soon followed - his colleague for 102 NC, Q. Lutatius Catulus, had performed less impressively against the Cimbri when they crossed the Alps. Once again Marius had to come to his rescue, defeating the Cimbri in the final battle of the war at Vercellae or the Raudine Plain (30 July 101 BC).