The battle of the Sambre (July 57 B.C.) was the most important battle of Caesar's campaign against the Belgae in 57 B.C. and saw his army recover after being ambushed to inflict a crushing defeat on three Belgic tribes led by the Nervii.
Over the winter of 58-57 B.C. the Belgae had created a massive army, reported by Caesar as being nearly 300,000 strong. The Belgae's biggest weakness was their supply system, and after an inconclusive battle on the Aisne the army dispersed back to its individual homelands. Their intention was to wait for Caesar to make his move and then bring the army back together to oppose him, but of course the Romans moved too fast for this plan to be effective, and a series of tribes were forced to surrender without offering any real resistance.
The only exceptions were the Nervii, Atrebates, Viromandui and Atuatuci, the most northerly of the Belgic tribes. Between them the Nervii, Atrebates and Viromandui had contributed 75,000 men to original Belgic army, so they were now in a much weaker position than at the start of the campaign, but they were still determined to fight. The Atuatuci were on their way to join the Nervii, but didn't arrive in time to take part in the battle of the Sambre.
By this stage in the campaign Caesar had clearly become overconfident. He was marching with his six experienced legions at the front, followed by the baggage and then by two new legions raised over the winter of 58-57 B.C. As the army approached the Sambre the cavalry and light troops were sent across the river to guard against the Nervii, while the lead six legions were all ordered to build that day's camp. No infantry screen was put in place to protect the legions while they were working on the camp.
The situation was perfect for an ambush. The main Nervii force was hidden in some woodland on the far side of the Sambre. The countryside on the south bank was divided up by tall, almost impenetrable hedges, which made it almost impossible for anyone to see what was happening on other parts of the battlefield.
The Roman cavalry soon ran into the Nervii and was defeated and driven back across the Sambre. The Belgae then charged across the river so fast that Caesar and his officers didn't have time to react. Only the professionalism of the six experienced legions saved them from immediate defeat. Without waiting for orders the legions formed up into their order of battle and prepared to fight.
Caesar's army formed up in the order it had been working on the camp with the ninth and tenth legions on the left, the eight and eleventh in the centre and the seventh and twelfth on the right. Two new legions formed over the winter of 58-57 B.C. were acting as the rear guard of the army, and didn't reach the battlefield until later. Each wing of the Roman army faced a different tribe – the Roman right was attacked by the Atrebates, the centre by the Veromandui and the left by the Nervii. The nature of the battlefield, crossed by tall impenetrable hedges, meant that each part of the battle developed separately, and Caesar was unable to exert much influence on the overall course of the battle.
The biggest influence on the events that followed was the relative strength of the three Belgic tribes. Caesar reported that the Nervii had promised to provide 50,000 men for the original army, the Atrebates 15,000 and the Veromandui 10,000. This meant that the Roman left was badly outnumbered while the right and centre were fighting on roughly equal terms.
The Nervii and their allies attacked all along the Roman line. On the right the Atrebates were quickly defeated, and were even pushed back across the Sambre while in the centre the Veromandui were forced back to the river.
The Roman left was much harder pressed. Caesar joined the twelfth legion, which by then had lost most of its officers, along with the standard of the fourth cohort. The legion was becoming dangerously compressed, making it hard for the soldiers to use their swords. To make things worse the success on the Roman right and centre had left their camp dangerously exposed, and it soon fell to the Nervii.
Caesar had joined the twelfth when it became clear that it was facing the main Belgic attack. As the crisis developed he led from the front, snatching a shield and placing himself in the front ranks, where he ordered the troops to spread out. Realising that the seventh legion was also hard pressed he ordered the two legions to form up together and effectively fight back-to-back.
Caesar had won some time for his left wing, but it was still hard pressed. Fortunately for Caesar reinforcements began to arrive. The two legions of the rearguard finally arrived on the battlefield. On the other side of the river Titus Labienus, one of Caesar's best lieutenants, had captured the Nervii camp. From this vantage point he was finally able to see what was happening on the Roman left, and he sent the tenth legion back across the Sambre to help Caesar.
The Nervii now found themselves facing five full legions. They probably still outnumbered the Romans by around two-to-one but the tide of the battle had changed. The Nervii made a desperate last stand, and hardly any of them escaped the battle. According to Caesar when the old men, boys and women surrendered to the Romans they stated that of their 60,000 men only 500 were still able to bear arms.
In the aftermath of the battle Caesar extended his protection to the surviving Nervii. They were allowed to return to their own territory and their neighbours were warned not to take advantage of their weakened condition. Caesar then moved on to deal with the Atuatuci, who had been on their way to join the Nervii when the battle took place.