In 55 BC, Caesar massacred reportedly 400,000 Usipetes and Tencteri. When a supplicatio was proposed in the senate, M. Cato moved instead that Caesar should be surrendered to the enemy. This paper offers a technical analysis of an episode which, while well known, has not always been well understood. What Cato proposed was deditio (surrender) on the grounds that Caesar had committed a breach of fides (breach of truce and/or mistreatment of ambassadors) and thus an offence against the gods. Deditio was a means of expiation, so that Caesar alone should suffer for his crime, as in the famous case of C. Hostilius Mancinus in 136. Cato’s motivations were obviously political, but the technical and religious nature of his allegations meant they could not be ignored. Caesar, I argue, felt obliged to respond, and does so in BG 4 within particular constraints imposed by the nature of fides and deditio.