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.
Diodorus Siculus
In the next section (books VII–XVII), he recounts the history of the world from the Trojan War down to the death of Alexander the Great. The last section (books XVII to the end) concerns the historical events from the successors of Alexander down to either 60 BC or the beginning of Julius Caesar's Gallic Wars. (The end has been lost, so it is unclear whether Diodorus reached the beginning of the Gallic War as he promised at the beginning of his work or, as evidence suggests, old and tired from his labours he stopped short at 60 BC.) He selected the name "Bibliotheca" in acknowledgment that he was assembling a composite work from many sources. Identified authors on whose works he drew include Hecataeus of Abdera, Ctesias of Cnidus, Ephorus, Theopompus, Hieronymus of Cardia, Duris of Samos, Diyllus, Philistus, Timaeus, Polybius, and Posidonius.