When Caesar's year as consul expired in 58 BC, he assumed the proconsulship of Cisalpine Gaul and Gallia Narbonensis, or, as it was originally known, Provincia, after which the Provençal region in southern France is named. There, removed from Rome and the intrigues of his opponents in the Senate, Caesar commanded an area that extended from the Alps to the Pyrenees and north almost to Lake Geneva, as well as the provincial legions that would give him the wealth and fame that came of conquest, and the military support upon which his political survival depended.
Narbonensis became increasingly Romanized under Caesar and, when he writes in the Bellum Gallicum that Gallia est omnis divisa in partes tres, it is not this province, but the country beyond, Tres Galliae or Gallia Comata, "Long-haired Gaul," to which he refers. The same year that Caesar became proconsul, the Helvetii, a Celtic tribe living in present-day Switzerland, attempted to migrate through the area and settle in Gaul proper. The migration was unforeseen, but it did provide Caesar the opportunity for an advantageous war. There had been incursions into Gaul before, when Germanic tribes encroached on the territory for the first time in 121 BC, and, although they were driven back across the Rhine (it was then that Provincia was renamed), there were other attacks, the last in 105 BC, before the intruders finally were defeated by Marius (Caesar's uncle by marriage).
Caesar refused the Helvetii permission to enter the province and, when they still made the attempt, defeated them and forced the remnant to return home (of 368,000 people, Caesar writes that 110,000 survived). The Seubi, a Germanic tribe, also make inroads into the province that year, but a pretext was found and they, too, were routed, despite Ariovistus, their king, having been declared a friend of Rome. So begins Caesar's commentary on the Gallic War (58-52 BC) and the justification for his eventual conquest of the whole of Gaul, a defeat which Plutarch calculates to have resulted in the death of one million Gauls and another million enslaved (Life of Caesar, XV.5; Life of Pompey, LXVII.10; also Pliny, VII.91ff, where he cites a figure of 1,192,000 people being killed).
In 55 BC, Pompey and Crassus secured for themselves the consulship in Rome, and Caesar's proconsulship was extended for another five years. Had it not been, Caesar was at risk of being recalled and losing the protection of the army. Still, if he was to keep his name before the plebs and justify to a hostile Senate that he remain in Gaul, there had to be new conquests. It was to his advantage, therefore, to argue for Roman control of Gaul as protection against further German intrusion and to characterize the Gallic tribes, themselves, as posing a threat.
And this Caesar does. Their temperament is described as impulsive and emotional, and, because they are so credulous, fickled and easily swayed. Excitable in character and quick to anger, they are portrayed as politically unpredictable, brave and easily roused to war, but also superstitious and given to intrigue, impetuous, and prone to panic if a stratagem fails.
In 57 BC, Caesar defeated a confederation of Belgic tribes in northwest Gaul and also received the surrender of the maritime tribes of Normandy and Brittany. Others fled across the channel to Britain. When the tribes of Armorica on the Breton peninsula rebelled the following year, they, too, were defeated, the tribal leaders killed and the survivors sold into slavery. Again, assistance had been summoned from Britain.
Realizing that "in almost all the Gallic campaigns the Gauls had received reinforcements from the Britons," Caesar now prepared to invade and reconnoiter that troublesome isle.
References: Caesar: The Conquest of Gaul (1982) translated by S. A. Handford (Penguin Classics); Plutarch: Fall of the Roman Republic (1972) translated by Rex Warner (Penguin Classics); Britannia: A History of Roman Britain (1987).