The unsuccessful siege of Gergovia (May 52 B.C.) was the only major setback suffered by an army led in person by Julius Caesar during the entire Gallic War.
At the outbreak of the Great Gallic Revolt the Gauls had hoped to prevent Caesar from rejoining his legions in northern Gaul. When this failed Vercingetorix attacked Gorgobina, a move that forced Caesar to pull his legions out of their winter quarters in an attempt to lift the siege. As the Romans moved south they captured a series of towns (Vellaunodunum, Cenabum and Noviodunum). Vercingetorix was forced to abandon the siege of Gorgobina, and after a minor cavalry clash at Noviodunum was forced, somewhat against his will, to assist with the defence of Avaricum. Despite his best efforts this town soon fell to the Romans, and was subjected to a ruthless sack and massacre of the population.
The fall of Avaricum came at the end of the winter of 53-52 B.C. and the improving weather convinced Caesar that he could risk a wider campaign. He split his army of ten legions in half. Four, under his most able lieutenant Labienus, were sent north into the lands of the Senones and Parisii, who at that point were the most northerly tribes to have rebelled. Caesar himself led the remaining six legions south to attack Gergovia, in the lands of the Arverni, Vercingetorix's own tribe.
Caesar's plan was disrupted by the first signs of trouble within the Aedui, his most loyal Gallic ally. This tribe was ruled by an annually elected magistrate, but this year two men, Convictolitanis and Cotus, had been elected by different factions and there was a danger of civil war. Caesar travelled to Decatia (modern Decize), in Aeduan territory, where he found in favour of Convictolitanis. Caesar then requested more cavalry and 10,000 infantry from the Aedui before returning to his legions.
Caesar's next problem was a natural obstacle – the River Allier – which was then too full to ford. Vercingetorix was guarding the west bank of the river, which Caesar would need to cross if he was to reach Gergovia. Eventually Caesar tricked the Gauls by sending four of his six legions on a noisy march down the river, while he remained in hiding with two legions. Once the Gauls had moved after the main force Caesar's men rebuilt one of the bridges over the river and crossed onto the west bank. Rather than risk a battle on Caesar's terms Vercingetorix retreated south to Gergovia, where he camped on a series of hills close to the town.
Gergovia itself was built in a strong position on a steep hill. On his arrival Caesar realised that he wouldn't be able to storm the city, and decided to prepare for a regular siege. Initially all six legions camped together, but after a few days Caesar decided to capture a small hill that he hoped would limit the defender's access to fresh water. Two legions were posted in a small camp on this hill, with the remaining four legions in the main camp on the plains. The two camps were linked by a double trench twelve feet wide which allowed the Romans to pass safely between them.
The Romans were not given the chance to conduct their regular siege. Convictolanis, Caesar's choice as chief magistrate of the Aedui, quickly decided to join Vercingetorix's revolt. He appointed Litavicus, one of his allies, to command the 10,000 infantry that was going to join Caesar. When this force was a few days march away from the Roman camp Litavicus claimed that the Romans had massacred every Aedui in their camp, and produced witnesses who claimed to have witnessed the events. The Aedui reacted predictably, killing the Roman envoys with the army and preparing to march to join Vercingetorix at Gergovia.
This first crisis was quickly defused. Eporedirix, a high ranking cavalry commander who was one of the men Litavicus claimed had been killed, discovered what was happening and informed Caesar. Caesar reacted by leading four of his six legions to intercept the 10,000 Aeduan infantry. When the two forces came face to face Caesar sent Eporedirix into the Aeduan camp, and Litavicus's plan promptly collapsed. He was forced to flee to Vercingetorix, and his men submitted to Caesar, who used them later in the siege.
Before his fall Litavicus had sent messengers back into Aedui territory to spread the story. Convictolanis used these stories to inflame a general revolt, which quickly spread throughout the tribe.
When this news reached Caesar he realised that he would have to abandon the siege of Gergovia and move north to reunite all ten of his legions. His problem was how to find to retreat without it actually looking like that was what he had done.
A chance for a minor success soon presented itself. For most of the siege the hill that contained the Gallic camp had been heavily defended, but one morning the Romans realised that most of the garrison had disappeared. Captured prisoners revealed that the Gauls were building fortifications on a different part of the hills to prevent the Romans from cutting them off. Caesar decided to attack the Gallic camp, a success that would allow him to retreat from Gergovia without loosing face. According to Caesar's account that was as far as the attack was meant to go – once the camp was captured they were to hold their ground and advance no further.
The legions involved in the attack quickly captured the Gallic camp, nearly taking one Gallic king prisoner. Caesar then gave the signal to halt, but only the tenth legion obeyed the order. Led by a number of centurions who Caesar states were driven by greed, the rest of the attacking force attempted to storm the walls. A small force led by the centurion Lucius Fabius even reached the top of the walls, but the Roman attack ended in chaos. Gallic reinforcements soon reached the crisis point, and the outnumbered Romans on the walls were cut off and killed. Caesar had sent his Aeduan allies on a diversionary attack to the right, but when they returned from this attack they were mistaken for hostile Gallic reinforcements, causing a panic. Only the tenth legion and some cohorts from the thirteenth legion prevented the chaos from turning into a rout and eventually the Romans were able to return to their camps. The failed attack on the walls cost them 46 centurions and nearly 700 men.
On the next day Caesar censured his men for their rashness and for disobeying his orders. He was still looking for a way to abandon the siege with some credit, and so after haranguing his men he led them out of their camp and the army formed up in order of battle. Vercingetorix refused to take the bait. Caesar portrayed this as his being unwilling to risk a battle against the superior Roman army, and on the following day led his men back up the Allier. A few days later they were able to ford the Loire and the entire army was reunited in the lands of the Senones.