The battle of the Aisne (57 B.C.) was Julius Caesar's first victory in his campaign against the Belgic tribes of modern Belgium. At the end of the first year of his Gallic War Caesar's army entered winter quarters well to the north of the Roman Province of Transalpine Gaul. The Belgae, the inhabitants of modern Belgium, were understandably worried about Caesar's intentions, and formed a league to resist any Roman attack. This gave Caesar an excuse to campaign against them, and in the spring of 57 B.C. he lead his army north.
According to Caesar the Belgae were able to raise an army of around 300,000 men. Even if this figure was greatly exaggerated, Caesar's 40,000 legionaries (in eight legions) and their auxiliaries were still badly outnumbered. Caesar did have two advantages – the professionalism of his legions and the poor Belgic supply system, which limited the amount of time they could keep such a large army together.
Caesar decided to attempt to split his enemies. Divitiacus, the leader of Caesar's allies the Aedui, was sent to attack the lands of the Bellovaci, in the hope that their 60,000-strong contingent would leave the main Belgic army. Learning that the main army was approaching his position, Caesar advanced to a bridge over the Aisne. Six cohorts, under the command of Q. Titurius Sabinus, were left on the south bank of the river with orders to build a fortified camp. The rest of the army crossed the river and occupied a hill on the north bank.
The Belgae's first move was to attack the town of Bibrax, eight miles from the Roman camp. The town held out for the first day of the attack, but it was clear that it would fall on the second day. The defenders of Bibrax managed to get a message to Caesar, and overnight he moved a force of Numidian and Cretan archers and Balearic slingers into the town. This convinced the attackers that they would no longer be able to take the place, and the Belgic army moved to a new camp two miles from the main Roman position.
Caesar was still badly outnumbered, and over the next few days he refused to risk a battle. The fighting was restricted to a number of minor cavalry engagements, which apparently convinced Caesar that it was worth risking a battle. The Roman camp was built on a gently sloping hill that was wide enough for the entire army to deploy, and that had steep enough sides to prevent the Belgae from easily attacking the Roman flanks. Caesar further protected his flanks by building ditches along the slopes and by building forts at each end of his main line. The forts were filled with Caesar's field artillery.
With their preparations complete the Romans came out of their camp and formed up in order of battle. The Belgae did the same, and for some time it appeared that a major battle was about to begin, but the two armies were separated by a small marsh. Whichever side moved first would inevitably become disorganised while crossing the marsh, giving their opponents an advantage. Neither Caesar nor Galba was willing to make that first move, and so after another minor cavalry engagement the Romans returned to their camp.
The Belgae now took the initiative. They attempted to cross the river using a nearby ford in preparation for an attack on the six cohorts on the south bank of the Aisne. Caesar responded by moving his cavalry, lightly armed Numidians, slingers and archers across the bridge. The lighter Roman troops so fast that they reached the ford while most of the Belgic army was still crossing the river. While the Roman cavalry dealt with those troops that were already on the south bank the missile troops prevented the rest of the army from crossing the river. Eventually the Belgae retreated to their camp.
By this point the Belgic army was running short of supplies, and news had reached them that the Aedui were approaching the lands of the Bellovaci. At a council of war their leaders decided that each contingent should return to its own country, but remain under arms. They would then wait to see which way Caesar moved next and then bring the army back together. This was a disastrous move. The retreat itself nearly turned into a rout, while Caesar was able to move at least as fast as the dispersing Belgic army. Over the next few weeks the Suessiones, Bellovaci and Ambiani all surrendered to the Romans. Only the Nervii and their allies continued to resist the Romans, and they too were forced surrender after suffering a heavy defeat on the Sambre.