Historically the Battle of Alesia was the turning point in the Gallic wars between Rome and the Gaulic tribes, but what if Gaul had won? The odds at Alesia were pretty even, Vercingetorix (Commander of the Gallic forces) outnumbered the Romans 4:1 and had a strong defensive position. Julius Caesar the Roman general's army was much bettered trained and disciplined and they had built ingenious siege works. Many historians consider Vercingetorix's decision to send all of his cavalry to get help to be the deciding factor, the lack of cavalry allowed the Romans to build and forage freely. In this timeline Vercingetorix sends most of his cavalry but keeps just enough to harass the Romans.
Send in the Cavalry
Two weeks into the siege of Alesia, Vercingetorix's forces are low on food so Vercingetorix makes the decision to send 1,500 of his 2,000 cavalrymen. Vercingetorix retains just enough of his cavalry to harass the Romans, when they started to build a second wall to defend against the relief force they were unable to finish it when the relief army led by Commius arrived, 120,000 strong. Immediately the Galls attacked, they poured through the gaps in the unfinished outer wall and flooded the Roman camp while Vercingetorix and his army charged through the town gates. Despite their defenses and the Romans were overrun and were massacred, 50,000 Romans fell in the battled and the remaining 10,000 surrendered and were promptly executed. Caesar's fate is often disputed, some sources say he was killed by a stray arrow from an unknown archer, others that he was killed when swarmed by Gallic soldiers, some claim he was killed by Vercingetorix in the heat of battle. The one thing everyone agrees on is that the Roman forces were completely annihilated, Caesar among them.
Gaul was united under Vercingetorix and one of Rome's best generals was dead along with 12 legions, Alesia was the last straw for the Roman senate who was convinced the conquest of Gaul was too costly and was now a lost cause.