The Romans respected and feared the Gallic tribes. In 390 BC, the Gauls had sacked Rome, which left an existential dread of barbarian conquest the Romans never forgot. In 121 BC, Rome conquered a group of southern Gauls, and established the province of Transalpine Gaul in the conquered lands. Only 50 years before the Gallic Wars, in 109 BC, Italy had been invaded from the north and saved by Gaius Marius only after several bloody and costly battles. Around 63 BC, when a Roman client state, the Gallic Arverni, conspired with the Gallic Sequani and the Germanic Suebi nations east of the Rhine to attack the Gallic Aedui, a strong Roman ally, Rome turned a blind eye. The Sequani and the Arverni defeated the Aedui in 63 BC at the Battle of Magetobriga.
Rising politician and general Julius Caesar was the Roman commander and agonist of the war. As a result of the financial burdens of being consul (the highest office in the Roman Republic) in 59 BC, Caesar had incurred significant debts. To strengthen Rome's position among the Gauls, he had paid substantial money to Ariovistus, king of the Suebi, to cement an alliance.
Caesar had four veteran legions under his direct command initially: Legio VII, Legio VIII, Legio IX Hispana, and Legio X. As he had been governor of Hispania Ulterior in 61 BC and had campaigned successfully with them against the Lusitanians, Caesar knew most, perhaps even all, of the legions personally.
His ambition was to conquer and plunder some territories to get himself out of debt. It is possible that Gaul was not his initial target, he may have been planning a campaign against the Kingdom of Dacia in the Balkans instead. However, a mass migration of Gallic tribes in 58 BC provided a convenient casus belli, and Caesar prepared for war.