After taking Avaricum and supplying his legions with badly needed sustenance, Caesar began to move against the main body of Gallic resistance. In the early summer Labienus was sent with 4 legions against the Senones and Parisii, and Caesar pursued Vercingetorix with the 6 remaining legions accompanied by Aedui auxilia and Germanic cavalry. Following the Allier River, Vercingetorix marched ahead of the Roman host, destroying bridges as they went to delay the pursuit. Near the hill fort of Gergovia, with favorable ground for a defensive stand, Vercingetorix stopped and prepared to meet Caesar.
After a 5 day march, the Romans engaged the Gallic cavalry just outside Gergovia. Pushing them back, Caesar moved into position against the enemy infantry. In surveying the field it was obvious that Vercingetorix was well positioned on favorable ground, but a hill adjacent to the main fort was only lightly guarded. During the night, the Romans took the lightly held hill with 2 legions and fortified their position. Rather than risk a full frontal assault, Caesar once again relied on his superior siege tactics. This time he ordered a double trench, 12 feet wide, to be constructed between the newly captured hill and his main camp. Intending to completely encircle Gergovia and starve the Gauls inside, Caesar was interrupted by trouble with his Gallic allies the Aedui.
Through the treachery of a chieftan by name of Litavicus, the Aedui were spurred to join the revolt by being told that Caesar had slaughtered Aedui hostages previously given up to him as a condition of peace. Caesar broke off the attempted siege of Gergovia with 4 legions, leaving 2 to hold the defenses against Vercingetorix. He met the Aedui some 25 miles away, and with the presence of his legions, the Aedui submit once again to Roman authority. Litavicus managed to flee to join Vercingetorix at Gergovia, but the Aedui were again in the Roman fold, albeit temporarily. While in the process of securing Aedui loyalty, however, Caesar received word from Gaius Fabius who was left in command at Gergovia that his two legions were under heavy attack. Fearful that he might lose the legions and his camp, Caesar ordered a hasty march, displaying his famous knack for uncanny speed on campaign. His 4 legions marched backed to Gergovia in just several hours overnight relieving the pressure on Fabius.
Upon returning, the Gauls had put themselves into a better position to resist Caesar's encirclement plan. He knew that the siege would be a failure, and the only way to win the battle would be to get Vercingetorix to come down from the high ground. Caesar ordered 1 legion to move into some woods below the town, seemingly as a decoy. The main Gallic force moved itself out of position from their camp on the high ground, leaving it exposed, and Caesar moved the bulk of his force to take advantage. The remaining defenders were pushed from their positions on favorable ground and Caesar accomplishing his goal, Caesar ordered a general retreat, likely to reform for battle in a better situation. Caesar reports however, that the bulk of his force either ignored or didn't hear the order, and with the defenders cleared, the Romans began to storm the walls of Gergovia. Encouraged by the potential for plunder and glory, the men attacked oblivious to the danger.
The main body of Gauls that had moved to watch the single legion that Caesar ordered into the woods returned to protect Gergovia. Joining the attack, and gaining favorable position once again, they pushed the Romans from the hill and sent them into flight. Caesar's vaunted 10th legion and parts of the 13th were positioned on level ground to intercept the pursuing Gauls and stem off a potential route. Seeing the Romans in a position to their advantage, Vercingetorix called off the pursuit and moved back up the hill. At the end of the battle, Caesar wrote that 46 centurions and 700 legionaries were missing (presumed killed). Angry with his men for disregarding his retreat order, he chastised them for arrogance, but ordered them back into position to offer battle again. The next day, the Roman lines were drawn again, daring Vercingetorix to attack, but despite some cavalry skirmishes the Gauls were not budging. Realizing that Gergovia would just have to end in his first humiliating defeat, Caesar built a bridge across the Allier, and retreated out of reach of the enemy.
Meanwhile, Labienus was busy against the Parisii and Senones. Also finding himself in precarious positions similar to that of Caesar, he was at first hard pressed to offer battle favorable to Roman tactics. Eventually, near Lutetia, the Gauls attacked Labienus but he smashed the Gallic right wing outflanking and chasing them off the field. In the battle, the Gallic commander Camulogenus was killed and the north central part of Gaul was largely subdued. The Romans secured supplies from Agendicum and marched swiftly to rejoin Caesar to consolidate their forces. Caesar hoped that a single large force would be better suited to defeat Vercingetorix and crush the rebellion in a single effort. However, by this time, the Aedui, likely inspired by the Gallic victory at Gergovia, threw their full support in with the revolt offering as many as 15,000 cavalry to Vercingetorix. At an official meeting of Gallic tribes, he was elected as overall commander of all the combined tribes, essentially making him the first king of Gaul.
Having fully united the Roman forces in the whole of Gaul, save for those of Lucius Caesar with 22 newly recruited cohorts assigned to the defense of various perimeter tribes, Caesar next called upon the Germanics for more support. Arriving quickly with a large force of cavalry, Caesar found their horses in poor shape and replaced them with horses from his own army. Caesar's army however, as it was recently combined and on the march, was heavily laden with its baggage train. Vercingetorix saw this as his best opportunity to attack a vulnerable Roman army on the march and in need to protect its baggage train, rather than in battle formation. Sometime in early September 52 BC, Vercingetorix attacked the Romans with his cavalry near Divio (modern Dijon) from both the front and rear of the Roman column. Caesar countered the Gauls with his Germanic cavalry and shattered them, sending them racing back to their own infantry lines. Caesar moved in hot pursuit, scattering resistance and the Gauls, having lost the bulk of their cavalry retreated. Vercingetorix moved his army into the heart of central Gaul, in Mandubii territory and they regrouped at the well fortified town of Alesia preparing to meet Caesar's coming siege