The Gallic Republic was forged within the many wars that began roughly around 692 AUC (62 BC) that took place in the eastern half of Gallia, involving incursions from Germanic tribes and the migration of the Helvetii tribe. The Suebi, a powerful Germanic tribe under the leadership of their king, Ariovistus, entered into an alliance with the Sequani, a Gallic people living north of the Helvetii, who lived north of lake Lemanus (Geneva), to assist in their war against the Aedui, a powerful tribe living west of the Sequani.
The Aedui were a tribe closely allied to the Roman Republic, and had a political system that may have been inspired by the Romans. Their chief magistrate, known as as the Vergobretus ('judgement-worker'), was elcted annually, and had power of life and death over all of the Aedui, but was forbidden to go outside of their territory. They had many client tribes who depended upon the Aedui. The Sequani, meanwhile, quarreled with the Aedui often, and the opportunity to use the Suebi to defeat them was far too appealing.
The Aedui sent Diviciacus, who had previously served as their Vergobretus, to Rome to seek help. A skilled orator and personal friend of both Cicero and Caesar, he pleaded fiercely for Roman military aid. Eventually, one legion was sent, the Legio IV Gemini, under the command of Lucius Aurelius Cotta. The news of the Roman support for the Aedui intimidated some of the chieftains under Ariovistus, but most of the Suebi and Sequani followed through with their invasion.
The Aedui army met with that of the invaders near the fort of Cabilionum, on the Arar (Saone) river. The Romans, who had arrived shortly before, had spent much of their time improving the fortifications of the site, as well as building several artillery engines with which to bombard the Suebi and Sequani. The bombardment was so effective that the invaders were obliged to attempt to cross over the river, to the southeast.
The Aedui pursued them fiercely, leading to panic and confusion among the enemy ranks. As the warriors boarded their boats, the Roman artillery was able to sink many of them, drowning countless Suebi and Sequani. The remainder of the force was able to flee the battle, returning to their homelands. Cotta, satisfied that the Romans had fulfilled their obligation, withdrew back to Italia, while the Aedui followed the Sequani back some ways up the Arar river, ensuring the the Aedui would control the traffic along the river.
After this war, the prestige of the Aedui was greatly increased, and many nearby tribes acquiesced to their primacy, including the Lemovices, Petrocorii, and the Bituriges Vivisci (cousins to the Bituriges Cubi, who already were clients of the Aedui). Thus, the Aedui held sway over much of central Gallia, down to the borders of Gallia Narbonensis, the Roman province, and west to the Atlantic Ocean, just north of Aquitania.
However, the Sequani were not content to lay defeated, and soon found a new ally against the Aedui. This time, it was the Helvetii, a fellow Gallic tribe, who, under their king Orgetorix, desired to move westward, so as to flee the incursions of the Germanic tribes pressing against their boarders. Orgetorix, who was of a clever and militant mindset, eagerly encouraged an alliance with the Sequani, agreeing to split much of the land of the Aedui between them.
The Helvetii were not inclined to be conquerors, however, and Orgetorix spent much effort and time persuading the leaders of the tribe to his point of view. Eventualy, by 695 AUC (59 BC), the Helvetians were ready to march, and began their trek westward, into the territory of the Aedui, soon joining up with the Sequani under Casticus.
As the assembled Helvetian host marched into the territory of the Aedui, a call to arms was raised among many of the gallic tribes allied to the Aedui. Even more were brought into the alliance through skillful diplomacy, with rumors spread that the Sequani and Helvetii planned to divide the territory of various other gallic tribes amongst themselves. The Aedui also requested more assistance from the Romans, but received little help, other than the Roman governor of Gallia Narbonensis blocking attempts by the Helvetii to march through Roman territory.
The first major battle between the two sides was again along the Arar river, approximately halfway between Cabilionum and Lugdunum. The Helvetii were eventually able to cross the river and managed to hammer the Aedui ranks, driving back many of the allied forces. The Aedui retreated back to their capital of Bibracte, while the allied tribes dispersed north and south to the fortifications along the Arar. The Helvetii and Sequani debated extensively on what course to proceed on, and eventually settled on pursuing the Aedui to Bibracte and taking the fortress, decapitating the leadership of their opponents.
However, while the invaders argued, the Aedui spent their time reinforcing the fortifications and preparing for the potential of a siege, efforts that were redoubled when scouts confirmed that their enemies were marching on Bibracte. Equally important was the assurance of the continuation of support from the other allied tribes, who were making cannily making sure to extract many promises from the embattled Aedui in their time of need. The allied tribes, for their part, harassed the progress of the advancing army, focusing their efforts on attacking scouting and foraging parties.
The Sequani and Helvetii reached Bibracte in mid June of 695 AUC (59 BC), and found the fortifications to be very impressive. Further, much of the surrounding forest had already been harvested for timber, depriving the attacks of much of the material they needed for circumvallation works. Still, they pressed on and were eventually able to encircle the fortress. No sooner had they done so, then the allied forces, which had contented themselves with harassing their efforts, began to attack in earnest.
Orgetorix and Casticus quickly scrambled to improve their fortifications to repel the attackers, bloodying both sides severely as they did so. By August, the besiegers were also besieged themselves, with daily sallies launched against them from both sides. At this point, Divicianus made great use of his diplomatic skills to sow dissent amongst the Helvetii, who had not been entirely enthused at the prospect of the war to begin with. By late September, enough were of the opinion that the war was entirely due to Orgetorix's ambitious scheming, and agreed to rebel, on the condition of amnesty.
On the morning of September 21st, the allied forces launched their largest attack yet, focusing all their efforts on the Sequani positions in the fortifications, leaving the Helvetii completely unmolested. Orgetorix responded by attempting to assist the beleaguered Sequani. However, the forces that he brought to bear were those that were loyal to him, as the disloyal commanders convinced him to leave them at their positions, in case the attack as a ruse. Persuaded, Orgetorix directly contributed to his defeat by completely separating the soldiers loyal to him from the greater body of the Helvetii.
Once the tribe was divided, the rebel leaders issued forth the signal, and opened their fortifications to the attackers, before turning on Orgetorix and Casticus and joining in the general attack on their positions. Orgetorix quickly realized what had happened and made a valiant effort to resist the assault, but soon came to the conclusion that victory was impossible, and committed suicide. Casticus was not so inclined and went down fighting. By the end of the day, the Sequani army had been virtually annihilated, and a fair portion of the Helvetii lay dead as well, aside from those that had rebelled, now under the leadership of Divico, leader of the Tigurini Helvetii.
However, the allied forces were also ravaged by battle and disease, and the question of what to do with the Helvetii remained. Adding to the discord were the demands of the various tribes allied to the Aedui that had been made in return for continued support. Many of these demands were mutually exclusive, and the entire network of alliances and treaties forged by the Aedui during the war seemed to be unraveling, as was typical of Gallic relations of the time.
Dumnorix, the brother of Divicanus, was able to break the deadlock of arguments by focusing on the outside threats that still face the Gallic tribes: The Germanic tribes and the Romans. Dumnorix was more inclined to passionate speeches and was no friend of the Roman Republic, while Divicanus was more sympathetic to the Romans and more patient with his negotiations. The two managed to compliment their respective talents and worked tirelessly to keep the tribes a cohesive force.
It was after much negotiation that a permanent council of the Gallic tribes was formed, to which the overwhelming majority of the allied tribes would send representatives. Taking cues from the Roman government, they set about negotiating and arguing how exactly their union would take shape. Though the debates continued throughout the year and for sometime after, the most important had already been resolved: there would be a unified state composed of Gallic tribes, who would settle their differences peacefully rather than through force of arms.
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Explanation
I don't have much to say about the unification of Gaul, other than that its not a complete process by any means. Largely just the tribes adjacent to and west of the Aedui have aligned themselves to the new government being formed around the nucleus that is the Aedui alliance.
The war itself is just a reinterpretation of the Gallic wars of our history, with the Romans largely left out, as their efforts have been focused largely eastward (particularly, in Dacia). Further, as Roman allies have effectively come out on top, there is much less of a reason for Roman interference, for the time being.
