Establishment of the Arvernian Empire and Conquest of Gaul

From around 100 CE, Greek and Carthaginian trading with the Gauls, especially the Arverni tribe, improved. Over time, they adopted more and more of their alphabet, urban systems, army style and culture.

In 135 CE, Christianity had reached the Arverni through the Rhône river. In 202 CE, the Arverni adopted Christianity, leading to the foundation of the Arvernian Empire. From 205 CE onwards, the Arvernian Empire grew more and more.

From 205-206 they started their first campaign by invading western territories, inhabited by small Gallic tribes, most knowingly the Helvetii. In 207, they sieged the city of Tolosa owned by the Tectosages. In 208, they defeated the Helvetii and small tribes inhabiting areas around them. Their next expansion was the Siege of Burdigala in 209 CE. They invaded some Biturigian (Cubi) Land west of Avaricum in 210 CE, eventually sieging the city in 211. They invaded the Pictones and their capital Lemonum in 212. From 213-214 CE, they attacked the Allobroges, eventually sieging Nicava (Greek: Nikaia) and connecting to the Mediterranean Sea in 214. In 215 CE, they defeated the Rest of the Allobroges by taking their capital Vienna and Geneva. Though, in 216, the Sequani and Aedui formed an alliance, hoping to stop the Arvernian expansion.

In 218, they defeated the Sequani, one of the most powerful tribes, taking their capital Vesontio. Afterwards in 220 they sieged Bibracte defeating once one of the most powerful tribes, the Aedui. They sieged Alesia in 221 and defeated the Lingones in 222. Though they mostly had stayed at peace with the Greek colonies in Gaul, they got invaded due to their wealth in 223 CE. In 224, they took more formerly Tectosagian land. They took Agedincum in 225 and also defeated the non-Celtic Aquitanians one year later. They invaded the region of Aremorica in 227. Furthermore, they sieged the Aulercian Capital Vindinium and Cenabum in 228. Afterwards, the Senones and various other tribes are pushed back behind the Sēquana (Seine) River. They then fortify the city of Lucotocia.

In 228, most of the surviving tribes form an alliance, known as the Belgian alliance to protect the Belgian tribes from the Gauls, though it was already too late. In 229 both armies clashed in Lucotocia; Senones, Remi, Nervii, Eburones and most other powerful Belgian alliance members are defeated and have suffered heavy losses. They retreated to the cities of Nemetocenna, Durocortorum and Aduatuca. Though these were all quickly overrun by the Arvernian military soon after. Eventually they defeated the Treveri and Leuci in 230. (for better visuality see https://www.youtube.com/watch?v=ZjoGh8F1CK8. May has some mistakes. Consider this article as the most accurate information source)