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.
Publius Licinius Crassus (son of triumvir)
In the first year of the Gallic Wars, Caesar and his Celtic Aeduan allies fought a defensive campaign against the Celtic Helvetii, and waged an offensive against the Germanic Suebi and their allies, led by Ariovistus. During the decisive battle against the Suebi that brought the first year of fighting to its conclusion, Publius Crassus was given command of the cavalry. In 58 BC, Caesar's cavalry auxiliaries numbered 4,000, comprising regiments from the Aedui and from the Gallic nations of Gallia Transalpina, already a Roman province. In Caesar's army, the primary strategic applications of cavalry were reconnaissance and intelligence gathering, conducted by detachments of "exploratores" (“scouts”) and "speculatores" (“spies”); communications; patrols, including advance parties and guard units on the flanks of the army on the march; skirmishing, and securing the territory after fighting by preventing the flight of surviving enemy. The cavalry charge was infrequent. In the opening stage of the war against the Helvetii, Caesar had retained a Gallic command structure; a lack of strategic coordination, exacerbated by conflicting loyalties, led to poor performance, which Caesar sought to correct with a more centralized command. Publius Crassus is the first Roman named as a cavalry commander in the war, and was perhaps given the task of restructuring.