The Battle of Alesia or Siege of Alesia (September 52 BC) was the climactic military engagement of the Gallic Wars, fought around the Gallic oppidum (fortified settlement) of Alesia in modern France, a major centre of the Mandubii tribe. It was fought by the Roman army of Julius Caesar against a confederation of Gallic tribes united under the leadership of Vercingetorix of the Arverni. It was the last major engagement between Gauls and Romans, and is considered one of Caesar's greatest military achievements and a classic example of siege warfare and investment; the Roman army built dual lines of fortifications—an inner wall to keep the besieged Gauls in, and an outer wall to keep the Gallic relief force out. The Battle of Alesia marked the end of Gallic independence in the modern day territory of France and Belgium.
Archeological and historical research confirms that the battle site was most probably atop Mont Auxois, above modern Alise-Sainte-Reine in France.
The event is described by Caesar himself in his Commentarii de Bello Gallico as well as several later ancient authors (namely Plutarch and Cassius Dio). After the Roman victory, Gaul (very roughly modern France) was subdued, although Gallic territories north of Gallia Narbonensis would not become a Roman province until 27 BCE. The Roman Senate granted Caesar a thanksgiving of 20 days for his victory in the Gallic War.
Background
In 58 BC, following his first consulship in 59 BC, Julius Caesar engineered his own appointment as proconsul (governor) of three Roman provinces by the First Triumvirate. These were Cisalpine Gaul (northern Italy), Illyricum (on the eastern coast of the Adriatic Sea) and Gallia Narbonensis (in southeastern France and the rest of France's Mediterranean coast). Although the proconsular term of office was meant to be one year, Caesar's governorship was for an unprecedented five years. He also had the command of four legions.
Caesar engaged in the Gallic Wars (58–50 BC), which led to his conquest of Gaul beyond Gallia Narbonensis. When the Helvetii, a federation of tribes from what is now Switzerland, planned a migration to the Atlantic coast through Gaul, Caesar went to Geneva and forbade the Helvetii to move into Gaul. While he went to Gallia Cisalpina to collect three other legions, the Helvetii attacked the territories of the Aedui, Ambarri, and Allobroges, three Gallic tribes, which called for Caesar's help. Caesar and his Gallic allies defeated the Helvetii. The Gallic tribes then asked for Caesar to intervene against an invasion by the Suebi, a Germanic tribe. Caesar defeated the Suebi and, to demonstrate Roman power, crossed the Rhine in 55 BC. In 57 BC he intervened in intra-Gallic conflicts and marched on the Belgae of northern Gaul. From then on he conquered the Gallic peoples one by one. His successes in Gaul brought Caesar political prestige in Rome and great wealth through the spoils of war and the sale of war captives as slaves.
Gallic existential concerns came to a head in 52 BC and caused the widespread revolt the Romans had long feared. The campaigns of 53 BC had been particularly harsh, and the Gauls feared for their prosperity. Previously, they had not been united, which had made them easy to conquer. But this changed in 53 BC, when Caesar announced that Gaul was now being treated as a Roman province, subject to Roman laws and religion. This was a subject of immense concern for the Gauls, who feared the Romans would destroy the Gallic holy land, which the Carnutes watched over. Each year the druids met there to mediate between the tribes on the lands considered the center of Gaul. A threat to their sacred lands was an issue that finally united the Gauls. Over the winter the charismatic king of the Arverni tribe, Vercingetorix, assembled an unprecedented grand coalition of Gauls.
Prelude
Caesar was still in Rome when news of the revolt reached him. He rushed north in attempt to prevent the revolt from spreading, heading first to Provence to see to its defense, and then to Agedincum to counter the Gallic forces. Caesar took a winding route to the Gallic army to capture several oppida for supplies. Vercingetorix was forced to withdraw from his siege of the Boii (allied to Rome) capital of Gorgobina. However, it was still winter, and Vercingetorix realised the reason Caesar had detoured was that the Romans were low on supplies. Thus, Vercingetorix set out a strategy to starve the Romans. He avoided attacking the Romans outright, and instead raided foraging parties and supply trains. Vercingetorix abandoned a great many oppida, seeking to only defend the strongest, and to ensure the others and their supplies could not fall into Roman hands. Once again, Caesar's hand was forced by a lack of supplies, and he besieged the oppidum of Avaricum where Vercingetorix had pulled back to.
Vercingetorix had originally been opposed to defending Avaricum, but the Bituriges Cubi had persuaded him otherwise. The Gallic army was camped outside the settlement. Even while defending, Vercingetorix wished to abandon the siege and outrun the Romans. But the warriors of Avaricum were unwilling to leave it. Upon Caesar's arrival, he promptly began construction of a defensive fortification. The Gauls continuously harassed the Romans and their foraging parties while they built their camp, and attempted to burn it down. But not even the fierce winter weather could stop the Romans, and a very sturdy camp was built in just 25 days. Siege engines were built, and Caesar waited for an opportunity to attack the heavily fortified oppidum. He chose to attack during a rainstorm, where the sentries were distracted. Siege towers were used to assault the fort, and artillery battered the walls. Eventually, the artillery broke a hole in the wall, and the Gauls were unable to stop the Romans from taking the settlement. The Romans then looted and raped the settlement; Caesar took no prisoners and claimed the Romans slew 40,000. That the Gallic coalition did not fall apart after this defeat is a testament to the leadership of Vercingetorix. Despite this setback, the Aedui were willing to revolt and join the coalition. This was yet another setback to Caesar's supply lines, as he could no longer get supplies through the Aedui (though the taking of Avaricum had supplied the army for the moment).
Vercingetorix now withdrew to Gergovia, the capital of his own tribe, which he was eager to defend. Caesar arrived as the weather warmed and fodder finally became available which somewhat eased supply issues. As usual, Caesar promptly set to building a fortification for the Romans. Caesar then set about capturing territory closer to the oppidum. What happened in the ensuing Battle of Gergovia remains somewhat unclear. Caesar claimed that he had just ordered his men to take a hill near to the oppidum, and that he then sounded a retreat. But no such retreat occurred and the Romans directly assaulted the settlement. Gilliver finds it likely that Caesar did not actually sound a retreat and that it was his plan all along to directly assault the settlement. Caesar's dubious claim is likely to distance himself from the ensuing and overwhelming failure of the Romans. The Roman assault ended in clear defeat as the Romans were greatly outnumbered. Caesar (whose self-reported casualty numbers are likely much lower than the actual amount) claims that 700 men died including 46 centurions. Caesar withdrew from his siege and Vercingetorix's victory attracted many new tribes to his cause. So too, however, did the Romans who convinced numerous Germanic tribes to join them.
Vercingetorix marched the army he had gathered thus far, mainly cavalry, to intercept Caesar. The two armies met in the Battle of the Vingeanne, where Caesar won the subsequent victory defeating Vercingetorix's cavalry.
Siege
With his cavalry routed Vercingetorix withdrew towards the Mandubii oppidum of Alesia, in what would become the siege of Alesia. After the poor performance at Gergovia, a direct assault on the Gauls by Caesar was no longer a viable solution. Thus, Caesar opted simply to besiege the settlement and starve out the defenders. Vercingetorix was fine with this, as he intended to use Alesia as a trap to conduct a pincer attack on the Romans, and sent a call for a relieving army at once. Vercingetorix likely did not expect the intensity of the Roman siege preparations. Although modern archeology suggests that Caesar's preparations were not as complete as he describes, it is apparent that Caesar laid some truly incredible siege works. Over the span of a month, some 25 miles of fortifications were built. They included a trench for soldiers, an anti-cavalry moat, towers at regular intervals, and booby traps in front of the trenches. The fortifications were dug in two lines, one to protect from the defenders, and one to protect from the relievers. Archeological evidence suggests the lines were not continuous as Caesar claims, and made much use of the local terrain, but it is apparent that they worked. Vercingetorix's relieving army arrived quickly, yet concerted coordinated attacks by both the defenders and relievers failed to oust the Romans.
Alesia was an oppidum on a lofty hill, with two rivers on two different sides. Due to such strong defensive features, Caesar decided on a siege to force surrender by starvation. Considering that about 80,000 men were garrisoned in Alesia, together with the local civilian population, this would not have taken long. To guarantee a perfect blockade, Caesar ordered the construction of an encircling set of fortifications, a contravallation, around Alesia. It was eleven Roman miles long (16 km or 10 modern miles, each Roman mile equal to 1,000 paces), and had 23 redoubts (towers). While work was in progress, the Gauls carried out cavalry sallies to disrupt the construction. Caesar placed the legions in front of the camp in case of a sortie by the enemy infantry and ordered his Germanic allies to pursue the Gallic cavalry.
Before the encompassing fortifications were complete and under cover of night, Vercingetorix sent out all his cavalry to rally the tribes to war and come to aid him at Alesia. When Caesar heard of this from deserters and captives, he dug a trench twenty pedes (6 metres, 19 modern feet) with perpendicular sides and built all the other works four hundred stades (probably 592 m, 1943 feet) away from that trench. The object of placing this trench so far away from the rest of the works was, as Caesar explained, that the manning of the entrenchment was not easy and, thus, this distance was a protection against surprise enemy advances at night or against javelins or other missiles being thrown at the Roman troops who were building the works during the day. Between this advance trench and the entrenchment, he dug two more trenches 15 pedes (4.45 m, 14.6 ft) wide and deep. He filled the inner one, where the ground was level with the plain or sank below it, with water from the river. Behind the three trenches he built a rampart riveted with palisades 12 pedes high (3.57 m, 11.7 ft). On top of this he built battlements (parapets with squared openings for shooting through) and breastwork (wooden screens at breast height to protect the defenders) with large horizontal pointed stakes projecting from the joints of the screens to prevent the enemy from scaling it. All round the works he set turrets at intervals of 80 pedes (24 m, 78 ft).
Some of the Roman soldiers had to go a considerable distance to get the timber for the construction of the works and grain to feed the troops. This reduced the number of troops at the Roman works. The Gauls made sorties with large forces to attack the works. Therefore, Caesar added further structures to the works to make them defensible by the reduced number of troops. Cut tree trunks were sharpened to create stakes. They were fastened at the bottom and sunk into a five pedes deep trench (1.5 m, 4.9 ft) with the boughs protruding from the ground. They were tied in rows of five so that they could not be pulled up without the attackers being impaled by the sharp stakes. Pits three pedes (0.9 m, 2.9 ft) deep which sloped inwards slightly to the bottom were dug in front of the stakes. They were placed in five intersected rows in the shape of a quincunx (an arrangement of five objects with four at the corners and the fifth at the centre). Tapering stakes, the thickness of a man's thigh, were sharpened at the top, hardened with fire, and sunk in the pits. They protruded from the bottom of the pit to a height of four fingers. Earth was pressed hard to a height of one foot from the bottom of the pit to make the stakes firm. The rest of the pit was covered with twigs and broken-off tree branches to hide the trap. Eight rows of this kind were placed three pedes (0.9 m, 2.9 ft). In front of these, one pes (0.3 m, 0.97 ft) stakes with iron hooks were sunk into the ground and scattered close to each other all over the field.
To prepare for the arrival of the Gallic relief forces, Caesar constructed an outer fortification (a circumvallation) with the same specifications but facing the opposite way as protection against the external attack by this relief force. It followed the most favourable ground and formed a circuit of 14 Roman miles (20.7 km, 12.86 modern miles).