The battle of Vesontio (September 58 B.C.) was the second major victory of Julius Caesar's military career and saw him defeat a large army of Germans led by Ariovistus, a Suebian chief who had crossed the Rhine some years earlier to intervene in a war between Rome's ally's the Aedui and the Sequani. Ariovistus had helped the Sequani win their dispute, but had then settled west of the Rhine, and by 58 B.C. ruled around 120,000 Germans and had taken over two thirds of Sequani territory.
Caesar's initial intervention in Gaul had been to stop the migration of the Helvetii from modern Switzerland to the west coast of France. In June 58 B.C. he won a decisive victory over the Helvetii at Bibracte, and forced them to return to their homeland. Caesar was then approached by a delegation of Gallic leaders who requested Roman aid against Ariovistus and his Germans. Caesar would not have needed much convincing to become involved – fifty years early the Romans had suffered a series of heavy defeats at the hands of the Cimbri and the Teutones, two tribes that had invaded Gaul and northern Italy and destroyed several Roman armies before their final defeat, and since then the Romans had dreaded a second German invasion.
Before moving against Ariovistus Caesar sent two embassies to him, each of which was rebuffed, not the behaviour that the Roman's expected from one of their 'friends'. Caesar then gave one of the first demonstrations of his ability to move quickly, advancing east to capture the Sequani capital of Vesontio (modern Besançon).
The two armies were now within striking distance of each other. The proximity of what was seen as a massive Germanic horde caused a panic in the Roman army, apparently the only time this happened during Caesar's wars in Gaul. Caesar gathered the centurions together and managed to restore order. Early on the next day the Romans left Vesontio on a circuitous route that was designed to bring them out onto open ground after a march of 50 miles. This route is normally believed to have been through the Belfort Gap towards Mulhouse and the Rhine, but if so the Roman's didn't reach very far along the route, for Caesar placed the eventual battle fifty miles away from the Rhine. This vagueness regarding the battle's actual location helps explain while it is normally known as the battle of Vesontio, despite happening seven days march from that town.
On the seventh day after the Romans left Vesontio Caesar was informed that the Germans were only 24 miles away. The rapid Roman advance had clearly unnerved Ariovistus, who now requested a meeting. This took place on an earth mount half way between the two armies, and ended in failure, with both leaders stating their case but neither willing to compromise.
Two days later Ariovistus attempted to arrange a second meeting, but was rebuffed. On the same day the build-up to the battle began, when the Germans moved their camp to a new position six miles from the Romans. On the next day Ariovistus moved past the Roman camp and took up a new position two miles to his west, in an attempt to prevent supplies reaching Caesar from the Sequani and the Aedui. On the following five days Caesar formed up his army in order of battle, but on each day the Germans limited themselves to a number of cavalry skirmishes. On the sixth day Caesar formed up in order of battle again, but this time he marched past the German camp, and while his first two lines remained ready to fight his third line constructed a new camp, 600 paces past the German camp. Ariovistus attempted to use his light troops to prevent this, but they were beaten off by the Romans. Once the camp was complete Caesar left two legions in this small camp while the other four remained in the first camp.
On the following morning Ariovistus once again refused to come out and fight, but in the afternoon he attempted to seize the smaller camp. This attack was repulsed, and the Romans finally discovered why the Germans had refused to fight – Ariovistus was obeying a divination that stated the Germans would not win if they fought before the new moon.
Caesar decided to take advantage of this by attacking the camp before the new moon, forcing the Germans to fight. On the following day he gathered his six legions at the larger camp while the auxiliaries were left to defend the smaller camp.
Caesar drew up his six legions in three lines, with the cavalry in reserve. Caesar took command of the Roman right, facing the weakest part of the German army, clearing hoping to defeat the German left and then turn on their right. Command of the cavalry was given to P. Crassus, the son of Caesar's colleague in the triumvirate. The Romans advanced towards the German camp, eventually coming so close that Ariovistus was forced to deploy his army and prepare for battle.
Ariovistus led a composite army, with contingents from seven different Germanic tribes – the Harudes, Marcomanni, Triboci, Vangiones, Nemetes, Sedusii and Suevi. As the Romans approached their camp each tribe formed up separately, with equal spaces between them. Ariovistus posted the chariots and wagons around the edge of the army, to discourage any potential deserters, and then advanced towards the Romans.
The battle began with a charge by both sides. As a result the gap between the lines closed so quickly that the Romans were unable to throw their javelins. At first the two sides engaged in hand-to-hand fighting, but according the Caesar the Germans soon formed into a phalanx (probably a shield wall), which allowed them to resist the Roman attack.
As Caesar had hoped the Roman right was soon victorious, but on the left the first two lines were hard pressed by the strongest part of the German army. Caesar gave P. Crassus credit for saving the situation by ordering the Roman third line to support their left. This decided the battle. With the Romans victorious along the entire line the Germans turned and fled, not stopping until they reached the Rhine, fifty miles to the east of the battlefield. Ariovistus was amongst the survivors, escaping across the river in a small boat, but his two wives and one of his daughters were amongst the dead and his other daughter was captured. It would be three years before Caesar faced the Germans again.