Gaul is the name given by the Romans to the territories where the Celtic Gauls (Latin Galli, French Gaulois) lived, including present France, Belgium, Luxemburg and parts of the Netherlands, Switzerland, Germany on the west bank of the Rhine, and the Po Valley, in present Italy. The ancient limits of Gaul were the Rhine River and the Alps on the east, the Mare Nostrum (Mediterranean Sea), the Po Valley and the Pyrenees on the south, and the Atlantic Ocean on the west and North.
Before the Roman conquest by Julius Caesar (58-51 BC), the name “Gaul” corresponded to a cultural and military area founded on a common religion and federations of peoples who though that they had a common origin. This common origin probably dates back to 8th century, when migrants groups of the Bronze Age Urnfield culture spread slowly across the area of the future territory of Gaul. About 390 BC, the Gauls invaded and sacked Rome. In 222 BC, Cisalpine Gaul (the region between the Alps and the Po Valley) was conquered by the Romans. The best description we know about the pre-Roman Gaul is in the first chapter of the Commentarii de Bello Gallico, of Caius Julius Caesar:
After Julius Caesar had conquered Gaul, the territorial organization of Gaul as part of the Roman Empire was concluded by Emperor Augustus from 27 to 12 BC: Respecting the ancient organization described by Julius Caesar, Augustus created three Roman Provinces: Gallia Belgica, Gallia Lugdunensis and Aquitania. In the south, the old Roman Provincia, to which Massilia was added, was renamed Gallia Narbonensis. The territories bordering the Rhine River were combined into two military areas, which under Domitian became the provinces of Upper and Lower Germania. For about 200 years the Roman peace was maintained, with the exception of some local revolts and civil troubles. The Germanic incursions of the 3rd century AD marked the end of this epoch.