The Gallic Wars were a series of military campaigns waged by the Roman proconsul Julius Caesar against several Gallic tribes. They lasted from 58 BC to 50 BC and culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the Roman Republic over the whole of Gaul. The wars paved the way for Julius Caesar to become the sole ruler of the Roman Republic. Caesar portrayed his invasion of Gaul as being a defensive pre-emptive action, most historians agree that the wars were fought primarily to boost Caesar's political career and to pay off his massive debts. Even so, Gaul was of significant military importance to the Romans, they had been attacked several times by native tribes indigenous to Gaul and further north. Conquering Gaul allowed Rome to secure the natural border of the river Rhine.