In the spring of 57 BC, Caesar was in Cisalpine Gaul attending to the administrative of his governorship. Despite, according to Caesar, cries of great thanks from various Gallic tribes for deliverance from the Suebi, discontent was growing. Word came to Caesar that a confederation of northern Gallic tribes was building to confront Roman presence in Gaul. It's important to note though, that Caesar by this time, had probably realized the only way to maintain the territories in eastern and southern Gaul, was to conquer the whole of the province. It's convenient for Caesar that these northern Belgic tribes would muster against the Romans for no reason, unless of course, they had reason to believe the Romans weren't done with expansion. Whatever the truth, Caesar hurried back to his legions, raising two new legions (the 13th and 14th) in the meantime, bringing his total to 8.
As Caesar arrived, likely in July 57 BC, the rumors of Belgae opposition proved true. Caesar moved quickly, surprising the Remi before they could join the opposition, and made fast allies of them. The Belgae, in reprisal against this began to attack the town of the Remi. At the Remi town of Bribrax, Caesar moved en masse to protect it from Belgae aggression. In an example of Caesar's brilliant ability to out march any known army, he surprised the enemy. With 8 legions the Romans crushed the attack in a hard fought affair. The victory was two fold for Caesar. It not only was a victory in the field, but a political and propaganda win as well. By defending his 'allies' from external aggression, he could now easily secure the necessary legalities to continue aggression against the Belgae. Though it would be another difficult campaign, this was exactly the sort of fortune that Caesar wanted.
After securing the town of Noviodunum and territory of the Suessiones, Caesar learned that the fearsome Nervii, with the Atrebates, Veromandui and Aduatuci, were forming against him on the opposite side of the Sambre River. The main part of the Roman army were in the midst of making camp along the river, while the two newest legions were bringing up the rear with the slow moving baggage train. Caesar sent out his cavalry to scout the situation, apparently unaware of the massing enemy preparing for ambush in the surrounding forests. The gathering Belgae, seeing Titus Labienus leading the Roman cavalry away, launched a complete surprise attack, storming the shallow river and pouncing on the unsuspecting Romans. The fighting was immediately desperate and the legions were hard pressed to maintain their ground. The Nervii and their allied tribes nearly surrounded the Romans, threatening the camp and the utter destruction of Caesar's army. Caesar's timely intervention, however, personally standing and fighting with his men, helped Roman discipline maintain itself.
In a nearly disastrous battle that could've changed the history of Europe, three things in particular kept the Romans from absolute defeat. Caesar's own personal intervention was important in stabilizing the men, the 2 legions with the baggage train arrived just in time to reinforce crumbling Roman lines, and the return of the Roman cavalry. Titus Labienus had taken the enemy camp in the midst of the fighting and was positioned at a vantage point where he could see the entire battle unfolding. He ordered Caesar's favorite 10th legion, no longer in danger itself, into a critical part of the enemy lines, relieving their distressed comrades. Soon after, the enemy lines broke and the Belgae warriors were in mass flight.
According to Caesar, though the numbers are assuredly in doubt, the Nervii surrendered and informed him that of 60,000 original warriors, only 500 remained. With the promise of no more aggression, he allowed them and other tribal combatants to return to their lands as subjects of Roman power.
The Aduatuci however, arriving late to the battle, fled intact to their lands and Caesar pursued. Arriving at the Aduatuci fort, where the tribe had walled it self in, Caesar showed a strategic brilliance in siege warfare that would be culminated at Alesia five years later. The Romans laid siege building a 15 mile wall 12 feet high around the fort and constructed siege weapons with which to ram the Aduatuci walls. Prior to complete destruction, Caesar offered peace and freedom to maintain their lands if only they would submit and give up their arms. Initially the tribe agreed, casting their weapons out of the fort and opening the gates to the legions. Caesar then suggests that after confirming the surrender, he ordered his men out of the fort for the night, to prevent unnecessary looting or injury to the Aduatuci. During the night however, thinking that the Romans would lower their guard in light of the surrender, the Aduatuci stormed the Roman lines hoping to catch them by surprise. The attempt failed and nearly 4,000 men were killed with the rest retreating to the fort. In the first of many perceived brutalities Caesar would be merciless with the Belgae who betrayed him. The next day the Romans stormed the fort capturing it quickly. He then claimed that 53,000 people were captured and sold into slavery, virtually wiping out the Aduatuci.
At this point, Publius Licinius Crassus returned, who had been sent against the Veneti, the Unelli, the Osismii, the Curiosolitae, the Sesuvii, the Aulerci, and the Rhedones of Brittany and Normandy. He informed Caesar that this northwestern section of Gaul was completely under Roman dominion. Though this assessment was not quite true, it served Caesar's purpose for the time being. He next negotiated cooperation with various bordering Germanic tribes to ensure the stability of his gains. Word was sent back to Rome, and despite opposition to his politics, Cicero himself pushed through an unusual Supplicatio, or public offering of thanks, for Caesar. A typical Supplicatio lasted only 5 days, and Pompey received 10 days for his conquest of the east. Caesar however received 15 days, marking a very strange concession to the man who was so reviled by the Senatorial conservatives. After putting most of his army into winter quarters, Caesar then moved to his third province of Illyricum to attend to matters there, while his lieutenant Galba, secured passes through the Alps by defeating resistant Seduni tribes.