In 55 B.C, the German tribes Usipetes and Tenctheri crossed the Rhine to the Roman occupied west bank and plundered corn they found there. Crossing back over they joined forces with the Sugambri tribe. When Caesar sent an embassy to the Sugambri and asked that the culprits be returned to him, the response was that the Rhine was the limit of Roman authority. Simultaneously, the Ubii tribe, an ally of Rome, asked for help against the Suebi tribe who were harassing them.
Caesar, deciding he had enough reasons to cross the Rhine, planned an expedition to attack the Sugambri.