Although Caesar portrayed this invasion as being a preemptive and defensive action, most historians agree that the wars were fought primarily to boost Caesar's political career and to pay off his massive debts. Still, Gaul was of significant military importance to the Romans, as these had been attacked several times by native tribes both indigenous to Gaul and farther to the north. Conquering Gaul allowed Rome to secure the natural border of the river Rhine.
The Gallic Wars are described by Julius Caesar in his book Commentarii de Bello Gallico, which is the most important historical source regarding the conflict.
(this pre-1923 publication has been converted from its original format for the Kindle and may contain an occasional defect from the conversion or from the original publication)