In 55 BC Caesar was busy preparing for his invasion of Britain. Whether his reasoning was retribution against those tribes that supported the Veneti and other opposing Gallic tribes as he himself claimed, the search to secure more metal sources such as tin and iron, or simply increasing his fame by being the first to cross, Caesar was prepared to make the crossing when events along the Rhine intervened.
Even while Caesar was in Luca meeting with Pompey and Crassus, word began to arrive of Germanic crossings of the Rhine. The Usipetes and Tenchteri tribes, both in a state of perpetual war with the powerful Suevi, began looking for safer land across the river.
Since the winter of 56/55 BC, the Germanics had displaced the Menapii tribe of Gauls, and Caesar was concerned that the incursions would lead to renewal of war in the region. According to him, he set out for the Rhine 'earlier than he was accustomed to do' with his legions, likely in the early spring of 55 BC. News arrived of further Germanic advances into the territory of the Eburones and the Condrusi, and Caesar was resolved for a fight. Caesar marched quickly and when he was within a few days march, the Germanics sent ambassadors to discuss the situation.
The general tone of the discussion was one of conciliation. The Germans claimed to only want peace and their own land away from the Suevi. Caesar refused their request to stay in Gaul but said he could arrange for settlement in the lands of the Ubii, who would be grateful for assistance against their mutual enemies, the Suevi. The invaders asked for 3 days to deliberate with their tribesmen, but Caesar refused, believing that they were only delaying long enough for their own cavalry to return from raids against the Ambivariti tribe. Caesar continued to march and when he arrived within 12 miles of the enemy camp, another embassy arrived to beg for more time. This time Caesar partially relented, ordering his cavalry which was scouting ahead of the main body, to resist committing aggression against the Germans.
Caesar's cavalry which numbered 5,000 soon found itself under attack, however. Apparently hoping to lure the Romans into complacency through negotiation, a much smaller force of Germanic cavalry launched a surprise attack on the Romans. With 800 men, the Germans caused considerable mayhem, killing 74 Romans before driving them all the way back to Caesar's main lines. While Caesar was preparing to launch a counter attack, he had an incredible stroke of luck.
A large Germanic contingent of tribal elders and leading men arrived to beg forgiveness for the treachery and to set matter right. Caesar, however, would have nothing to do with it and ordered the Germans seized and held. A full scale assault was then launched on the German camp and according to Caesar, 430,000 leaderless German men, women and children were assembled. The Romans butchered indiscriminately, sending the mass of people fleeing to the Rhine, where many more succumbed to the river. In the end, there is no account of how many were killed, but Caesar also claims to have not lost a single man. Of those that survived, many stayed with the Romans in service rather than face the angry Gauls or the Suevi in their depleted state.
With the situation secure on the Gallic side of the river, Caesar decided it was time to settle the matter with the aggressive Germans, lest they invade again. The Ubii alone, still seeking help against the Suevi, welcomed Roman intervention. It was decided, in order to impress the Germans and the Roman people that bridging the Rhine would have the most significant effect. By June of 56 BC, Caesar became the first Roman to cross the Rhine into Germanic territory. In so doing, a monstrous wooden bridge was built in only 10 days, stretching over 300 feet across the great river. This alone assuredly, impressed the Germans and Gauls, who had little comparative capability in bridge building. Within a short time of his crossing, nearly all tribes within the region sent hostages along with messages of peace.
Only the Sigambri resisted, fleeing their towns rather than submit to Caesar. The Romans made an example of them by burning their stores and their villages before receiving word that the Suevi were beginning to assemble in opposition. Caesar, rather than risk this glorious achievement in a pitched battle with a fierce foe, decided that discretion was the better part of valor. After spending only 18 days in Germanic territory, the Romans returned across the Rhine, burning their bridge in the process. With that short diversion, Caesar secured peace among the Germanics, as the Suevi remained relatively peaceful for some time after, and secured a crucial alliance with the Ubii. His rear secured, Caesar looked for another glorious Roman 'first' and moved his body north to prepare for the invasion of Britain.