The territory of modern Switzerland was a part of the Roman Republic and Empire for a period of about six centuries, beginning with the step-by-step conquest of the area by Roman armies from the 2nd century BC and ending with the Fall of the Western Roman Empire in the 5th century AD. The mostly Celtic tribes of the area were subjugated by successive Roman campaigns aimed at control of the strategic routes from Italy across the Alps to the Rhine and into Gaul, most importantly by Julius Caesar's defeat of the largest tribal group, the Helvetii, in the Gallic Wars in 58 BC. Wikipedia