A greater danger than Rome had experienced since the time of Hannibal now threatened the state. Vast numbers of barbarians, such as spread over the south of Europe in the later times of the Roman Empire, had collected together on the northern side of the Alps, and were ready to pour down upon Italy. The two leading nations of which they consisted are called Cimbri and Teutones, of whom the former were probably Celts and the latter Germans, but the exact parts of Europe from which they came can not be ascertained. The whole host is said to have contained 300,000 fighting men, besides a much larger number of women and children. The alarm at Rome was still farther increased by the ill success which had hitherto attended the arms of the Republic against these barbarians. Army after army had fallen before them. The Cimbri were first heard of in B.C. 113, in Noricum, whence they descended into Illyricum, and defeated a Roman army under the command of Cn. Papirius Carbo. They then marched westward into Switzerland, where they were joined by the Tigurini and the Ambrones. They next poured over Gaul, which they plundered and ravaged in every direction. The Romans sent army after army to defend the southwestern part of the country, which was now a Roman province; but all in vain. In B.C. 109 the Consul M. Junius Silanus was defeated by the Cimbri; in B.C. 107 the Tigurini cut in pieces, near the Lake of Geneva, the army of the Consul L. Cassius Longinus, the colleague of Marias, who lost his life in the battle; and shortly afterward M. Aurelius Scaurus was also defeated and taken prisoner. But the most dreadful loss was still to come. In B.C. 105 two consular armies, commanded by the Consul Cn. Mallius Maximus and the Proconsul Cn. Servilius Cæpio, consisting of 80,000 men, were completely annihilated by the barbarians: only two men are said to have escaped the slaughter.
These repeated disasters hushed all party quarrels. Every one at Rome felt that Marius was the only man capable of saving the state, and he was accordingly elected Consul by the unanimous votes of all parties while he was still absent in Africa. He entered Rome in triumph, as we have already said, on the 1st of January, B.C. 104, which was the first day of his second Consulship. Meantime the threatened danger was for a while averted. Instead of crossing the Alps and pouring down upon Italy, as had been expected, the Cimbri marched into Spain, which they ravaged for the next two or three years. This interval was advantageously employed by Marius in training the new troops, and accustoming them to hardships and toil. It was probably during this time that he introduced the various changes into the organization of the Roman army which are usually attributed to him. Notwithstanding the sternness and severity with which he punished the least breach of discipline, he was a favorite with his new soldiers, who learned to place implicit confidence in their general, and were delighted with the strict impartiality with which he visited the offenses of the officers as well as of the privates. As the enemy still continued in Spain, Marius was elected Consul a third time for the year B.C. 103, and also a fourth time for the following year, with Q. Lutatius Catulus as his colleague. It was in this year (B.C. 102) that the long-expected barbarians arrived. The Cimbri, who had returned from Spain, united their forces with the Teutones. Marius first took up his position in a fortified camp upon the Rhone, probably in the vicinity of the modern Arles; and as the entrance of the river was nearly blocked up by mud and sand, he employed his soldiers in digging a canal from the Rhone to the Mediterranean, that he might the more easily obtain his supplies from the sea. Meantime the barbarians had divided their forces. The Cimbri marched round the northern foot of the Alps, in order to enter Italy by the northeast, crossing the Tyrolese Alps by the defiles of Tridentum (Trent). The Teutones and Ambrones, on the other hand, marched against Marius, intending, as it seems, to penetrate into Italy by Nice and the Riviera of Genoa. Marius, anxious to accustom his soldiers to the savage and strange appearance of the barbarians, would not give them battle at first. The latter resolved to attack the Roman camp; but as they were repulsed in this attempt, they pressed on at once for Italy. So great were their numbers, that they are said to have been six days in marching by the Roman camp. As soon as they had advanced a little way, Marius followed them; and thus the armies continued to march for a few days, the barbarians in the front and Marius behind, till they came to the neighborhood of Aquæ Sextiæ (Aix). Here the decisive battle was fought. An ambush of 3000 soldiers, which Marius had stationed in the rear of the barbarians, and which fell upon them when they were already retreating, decided the fortune of the day. Attacked both in front and rear, and also dreadfully exhausted by the excessive heat of the weather, they at length broke their ranks and fled. The carnage was dreadful; the whole nation was annihilated, for those who escaped put an end to their lives, and their wives followed their example. Immediately after the battle, as Marius was in the act of setting fire to the vast heap of broken arms which was intended as an offering to the gods, horsemen rode up to him, and greeted him with the news of his being elected Consul for the fifth time.
The Cimbri, in the mean time, had forced their way into Italy. The colleague of Marius, Q. Lutatius Catulus, despairing of defending the passes of the Tyrol, had taken up a strong position on the Athesis (Adige); but, in consequence of the terror of his soldiers at the approach of the barbarians, he was obliged to retreat even beyond the Po, thus leaving the whole of the rich plain of Lombardy exposed to their ravages. Marius was therefore recalled to Rome. The Senate offered him a triumph for his victory over the Teutones, which he declined while the Cimbri were in Italy, and proceeded to join Catulus, who now commanded as Proconsul (B.C. 101). The united armies of the Consul and Proconsul crossed the Po, and hastened in search of the Cimbri, whom they found to the westward of Milan, near Vercellæ, searching for the Teutones, of whose destruction they had not yet heard. The Cimbri met with the same fate as the Teutones; the whole nation was annihilated; and the women, like those of the Tentones, put an end to their lives. Marius was hailed as the savior of the state; his name was coupled with the gods in the libations and at banquets; and he received the title of third founder of Rome. He celebrated his victories by a brilliant triumph, in which, however, he allowed Catulus to share.
During the brilliant campaigns of Marius, Sicily had been exposed to the horrors of a second Servile War. The insurrection broke out in the east of the island, where the slaves elected as their king one Salvius, a soothsayer. He displayed considerable abilities, and in a short time collected a force of 20,000 foot and 2000 horse. After defeating a Roman army he assumed all the pomp of royalty, and took the surname of Tryphon, which had been borne by a usurper to the Syrian throne. The success of Salvius led to an insurrection in the western part of the island, where the slaves chose as their leader a Cilician named Athenio, who joined Tryphon, and acknowledged his sovereignty. Upon the death of Tryphon, Athenio became king. The insurrection had now assumed such a formidable aspect that, in B.C. 101, the Senate sent the Consul M. Aquillius into Sicily. He succeeded in subduing the insurgents, and killed Athenio with his own hand. The survivors were sent to Rome, and condemned to fight with wild beasts; but they disdained to minister to the pleasures of their oppressors, and slew each other with their own hands in the amphitheatre.