Switzerland and the Roman Empire
The population of Switzerland before the conquest by the Romans in the last decades before Christ consisted of Celtic tribes. Geneva was already occupied since 120 BC as most northern city of the Allobroges and Roman province of Gaul Transalpine, the regions Languedoc-Roussillon, Provence-Alpes-Côte d’Azur and Rhône Alpes, except the independent city state of Massilia (Marseille). Emperor August reorganised this territory into the province Narbonensis in 22 BC with Narbo as provincial capital, including lacus lemanus (Lake of Geneva) and Geneva.
A special event took place in 58 BC, when the Celtic tribes of Helvetii and Raurici, inhabiting the region of Basel, Jura and the Swiss Plateau, immigrated to Gaul. An estimated 300 000 men, women, children were on the move, leaving behind their settlements. The reasons are unknown, but the pressure of Germanic tribes the tribes of Raetii is the most likely reason.
Caesar (governor of Gallia Narbonensis at this time) stopped this immigration/invasion by a decisive victory at Bibrace (Mont-Beuvray), close to Autun in 58 BC. Caesar feared the vacuum left in Switzerland and an invasion of Germanic tribes. The Helvetii and Rauraci were sent back to their homelands and became allies (foederati) of Rome. Caesar founded two Roman cities in this region in order to control the status quo. He founded Colonia Julia Equestris (Nyon) in the territory of the Helvetii, and the Colonia Raurica (Augst) in the territory of the Raurici. Many other settlements of Helvetii and Raurici should develop into Roman towns later on (for example Lousonna (Lausanne), Petinesca (Studen) or Eburodunum (Yverdon). Mont-Vully (Lake of Morat) is known because of the archaeological the finds of a Celtic oppidum.
The other parts of Switzerland remained formally independent until 15-13 BC, when a military campaign subdued the tribes of Nantuates, Seduni, Uberi and Veragri, inhabiting Vallis, and the Raeti, inhabiting the eastern part of Switzerland. The territory of the tribe of the Salasses on the other side of Mont-Iovis (mountain of Jupiter or the Grand Saint-Bernard) had already been conquered in 25 BC, when the Colonia Augusta Praetoria (Aosta) was created.
Many Celtic settlements became Roman towns or the Romans founded new cities (ex novo), for example Curia (Chur), Turicum (Zürich), Vindonissia (Windisch), Aqua Helveticae (Baden), Tenedo (Zurzach), Aventicum (Avenches), Salodurum (Solothurn), Vitudurum (Oberwintherthur), Tasgaetium (Eschenz) or Forum Claudii Vallensium (Martigny). The Roman expansion didn’t stop there, but reached southern Germany as well.
Emperor August and his successors reorganized these territories various times. Most relevant was the creation of the provinces Germania Superior (northern Switzerland (Avenches as capital of the Helvetii, Nyon and Augst, Elzas and the Franche-Comté). In the east he founded the province Raetia (Augst, Augusta Vindelicum as capital) and in the south the province Alpes Poeninae (Vallis). The neighbouring province Alpes Graiae was on the other side of the Small Saint-Bernard and had Aime (Forum Claudii Centronum) as capital. Geneva belonged to the Province Narbonensis and Lyon (Lugdunum) was the provincial capital of Lugdunensis (and of the Tres Galliae: Belgica, Lugdunensis and Aguitania).
This should remain the situation until 260 AD, when Germanic invasions swept over the region after a relatively peaceful period of 250 years. The Pax Romana was over.