Battle of Alesia
Caesar engaged in the Gallic Wars (58–50 BC), which led to his conquest of Gaul beyond Gallia Narbonensis. When the Helvetii, a federation of tribes from what is now Switzerland, planned a migration to the Atlantic coast through Gaul, Caesar went to Geneva and forbade the Helvetii to move into Gaul. While he went to Gallia Cisalpina to collect three other legions, the Helvetii attacked the territories of the Aedui, Ambarri, and Allobroges, three Gallic tribes, which called for Caesar’s help. Caesar and his Gallic allies defeated the Helvetii. The Gallic tribes then asked for Caesar to intervene against an invasion by the Suebi, a Germanic tribe. Caesar defeated the Suebi. In 57 BC he intervened in intra-Gallic conflicts and marched on the Belgae of northern Gaul. From then on he conquered the Gallic peoples one by one. His successes in Gaul brought Caesar political prestige in Rome and great wealth through the spoils of wars and the sale of war captives as slaves.
Tetricus I
The Gallic Empire is the historiographic name given to a state composed of the Roman provinces which made up Britannia, Hispania, and Gaul, which broke away from the Roman Empire during the reign of Emperor Gallienus. Gallienus had become emperor after his father, Emperor Valerian, was captured by the Sassanids in 260; his rule was part of the Crisis of the Third Century (235–284), a period of intense political and military power struggles. Gallienus was overwhelmed by numerous issues, including several usurpers, and barbarian attacks in the Balkans and along the Rhine — one attack by the Franks pushed as far as Tarraco (modern-day Tarragona) in Hispania. Because Gallienus was unable to prevent the raids, Postumus, a military commander on the Rhine frontier, rose up and declared himself emperor; at about the same time he assassinated Saloninus, Gallenius' son and co-emperor, in Colonia (modern-day Cologne). Postumus focused on defending the Gallic Empire, and, in the words of ancient Roman historian Eutropius:
Gallienus attempted to invade the Gallic Empire twice, but was repulsed both times, forcing him to acquiesce in the secession. Although he was unable to conquer the Gallic Empire, Gallienus did ensure that the Roman Empire was defended; posting Aureolus, a military commander, in northern Italy, to prevent Postumus from crossing the Alps. Postumus was killed by his own soldiers in 269 in Mogontiacum (modern-day Mainz) while putting down a revolt by the usurper Laelianus, because he refused to allow them to sack the city. After the army killed Postumus, they elected Marcus Aurelius Marius, an officer, as Gallic Emperor. While some ancient sources hold that Marius reigned for only two days before being killed by Victorinus, who had served as "praetorian prefect" (commander of the praetorian guard) under Postumus, the quantity of coins issued by Marius indicate that he must have served for a longer time, a period of roughly three months. Victorinus declared himself emperor in mid-269 in Augusta Treverorum (modern-day Trier), two days after killing Marius. Victorinus' rule was recognized by the provinces of Britannia and Gaul, but not by those of Hispania.