The siege of the Atuatuci tribe (September 57 B.C.) was the final major victory during Julius Caesar's conquest of the Belgae. Earlier in the year the Belgae had gathered a massive army to oppose the Romans, but after an inconclusive clash on the Aisne River that army had dispersed and each tribe had returned to its own territory. Caesar advanced through the Belgic territories, forcing the surrender of a series of tribes, but four, led by the Nervii, decided to fight on. In July the Nervii, Atrebates and Viromandui attacked Caesar's army as it was building a camp on the Sambre River, and were defeated with very heavy loses.
This only left the Atuatuci tribe, who had been on their way to join the Nervii when the battle took place. Realising that they could no longer hope to beat the Romans in an open battle the Atuatuci retreated into one of their fortified cities, described but not identified by Caesar. The town was surrounded on all sides by high rocks and cliffs. The only approach was up a gentle slope 200 feet wide which was defended by high double wall. The town is sometimes identified with Mont Falise, to the north of modern Huy on the Meuse River.
The Roman siege of the town was a major undertaking. Caesar reports having constructed a wall twelve feet high and fifteen miles in circuit around the town, but it was the construction of a large siege tower that convinced the Atuatuci to surrender, or rather the moment when they realised that the tower was moving towards the town. The Atuatuci agreed to hand over all of their weapons and in return Caesar promised to accept their surrender and take them under his protection.
On the night after making their surrender the Atuatuci attempted to break through the line of Roman fortifications, using weapons they had concealed in the town. The attack failed after the Romans had inflicted 4,000 casualties on the Atuatuci. The survivors retreated back into the town. On the following day the Romans occupied the town, which was now effectively undefended. Having broken the turns of their surrender, the 53,000 inhabitants of the town were sold into slavery.
The defeat of the Atuatuci ended the first campaign against the Belgae. At about the same time P. Crassus reported that the tribes of the Atlantic coast had submitted to Roman authority, and for a short period it looked as if Caesar had conquered all of Gaul in two campaigns.