Just as Marius was coming to power as Consul in 107 BC, a major migration by Germanic (perhaps Celtic) Cimbri was causing consternation along Rome's northern frontier. Apparently under threat of starvation from poor harvests and from external threats by other tribes, the Cimbri were on the move looking for new, more promising land. By 113 BC, the Germanics made their first appearance in Roman written history. These movements, and associated great losses in the Roman army stood as the main reason for Marius' military reforms, and not some great advocacy for the plebes, as the people of the time generally believed.
There is some evidence that the Germanics wanted little to do with the Romans, and that they simply sought safe passage to better lands. Others argue that they were an aggressive army looking for plunder. The Roman generals of the time, ambitious and politically motivated in a time of great change and opportunity for personal glory, may very well have provoked the Cimbri at every step. Regardless, the Cimbri did wander the Danube region for several years, involved in a number of engagements with local Celts. At some point a Roman army was sent to meet them in Noricum, modern Carinthia. Under Gnaeus Paprius Carbo, the Romans were routed and sent scrambling home (112 BC), while the Cimbri continued to move west towards Gaul.
After the defeat of Carbo, the Cimbri crossed the Rhine and threatened territory belonging to the Roman allied Allobroges. Tribal leaders attempted to negotiate land rights for their people, but all such requests were denied. By 109 BC, the Romans sent another force under the Consul Marcus Junius Silanus but again were soundly defeated, losing as many as 30,000 men. The Cimbri, however, not showing any desire to invade or cause trouble, went about their own business, looking for land in Gaul. In 107 BC another Roman army under the command of Longinus met up with the Cimbri near modern Tolosa. In addition to fresh recruits Longinus also led the veterans of Metellus' army from Africa, whose ultimate defeat along with the earlier losses, forced Marius to recruit from the Roman head count. Longinus was initially successful, but was eventually caught in an ambush. Killed in action, his subordinate, Laenas was forced to surrender his position and return to Rome with fewer than 4,000 survivors.
As if the crisis were not dire enough, the following two years were more disastrous still. In 106 BC, Quintus Servilius Caepio marched a fresh army towards Tolosa to enact revenge. When he arrived he was sidetracked by the discovery of the infamous 'Gold of Tolosa', a vast treasure. Winning a minor engagement, he let the Germanics move off, while he secured the treasure and prepared it to be returned to Rome. While en route, it 'mysteriously' disappeared, and the Caepio family suddenly became very wealthy and was the target of suspicion and accusations from that point on.
While he sat idle, the Senate was apparently unsatisfied with Caepio's performance and authorized another army to be raised. This time, a force of over 6 legions was hastily prepared under Mallius Maximus, and he was given imperium over both armies. He marched to join Caepio, but Caepio, feeling that Mallius was inferior in social position, refused to obey or join his command. Bitter in-fighting between the two men, and armies, would prove to be disastrous. In 105 BC, the Cimbri returned and came across the Romans arranged in two separate camps, with two full armies functioning completely independent of one another. At the battle of Aurasio (modern Orange), the Cimbri crushed both Roman armies, killing nearly 80,000 men while sustaining minimal losses of their own.
Despite this monumental victory, and the opening of the doorway to invade Italy, the Cimbri were still only interested in finding new land. They then divided their force, with some remaining in southern Gaul, but with the bulk moving on towards the Pyrenees and Spain. Bitter resistance by Celtibereans in Spain would eventually force the Germanics to return, but for the time being, Rome was granted a brief respite.
It was at this time that the opportunistic Marius returned to Rome to celebrate his triumph over Jugurtha. Rome, feeling the pinch of several successful military disasters, essentially granted complete military authority to Marius. In a breach of the Roman Republican constitution, 104 BC saw his election as Consul for the second time in only a few short years. The law required at least 10 years intervals between elections as Consul, but his election was proof of the faith that both the Senate and the people had in Marius' ability. A generally unpopular figure among the Senate, he was elected to an unprecedented 5 straight terms as Consul from 104 to 100 BC in order to deal with the Germanic threat.
In 104 BC, however, the Cimbri had moved on, and Marius spent the time reforming his legions, building roads and generally improving the condition of the provincial public works. Within 2 years, the Cimbri had joined up with more Germanics, including the Teutons, Helvetii and Ambrones. Failing to win new land in Spain from the Celts, they returned to what they may have thought would be easier adversaries in the Romans. In 102 BC, the Cimbri moved around the Alps to the eastern side of Italy preparing to invade. The Teutones moved to the west and followed the Alps south along the coast into Italy. Marius caught the Teutones and Ambrones at the battle of Aquae Sextiae late in the year 102 BC. This time, under competent Roman command, the Germanics were annihilated, and the Romans could focus on their other enemy, the Cimbri.
By early 101 BC, the Cimbri moved down from the Alps and started to press into eastern Italy. An army, technically under the command of Q. Lutatius Catulus, but practically led by Marius' subordinate Sulla, met the Cimbri at Vercellae. Again, the Germanic invaders were crushed with losses approaching 100,000. 2 great Germanic tribes were nearly routed completely from historical existence, and the 3 Roman commanders bickered over who could claim the victory. In the end, Marius shared a joint triumph with Catulus, but it was Marius who was heaped with the credit by the Roman people, and he was named the 'savior of Rome'. Perhaps, even more significant than the victory, was the political and personal impact. While Marius, without an enemy to fight, would soon prove his inability as a politician, the personal rivalry between Marius and Sulla grew ever more fractured.