The Gauls were embittered at being forced to feed the Roman troops over the winter. The Romans sent out officers to requisition grain from the Veneti, a group of tribes in northwest Gaul, but the Veneti had other ideas and captured the officers. This was a calculated move: they knew this would anger Rome and prepared by allying with the tribes of Armorica, fortifying their hill settlements, and preparing a fleet. The Veneti and the other peoples along the Atlantic coast were versed in sailing and had vessels suitable for the rough waters of the Atlantic. By comparison, the Romans were hardly prepared for naval warfare on the open ocean. The Veneti also had sails, whereas the Romans relied on oarsmen. Rome was a feared naval power in the Mediterranean, but there the waters were calm, and less sturdy ships could be used. Regardless, the Romans understood that to defeat the Veneti they would need a fleet: many of the Venetic settlements were isolated and best accessible by sea. Decimus Brutus was appointed prefect of the fleet.

Caesar wished to sail as soon as the weather permitted and ordered new boats and recruited oarsmen from the already conquered regions of Gaul to ensure the fleet would be ready as soon as possible. The legions were dispatched by land, but not as a single unit. Gilliver regards this as evidence that Caesar's claims the prior year that Gaul was at peace were untrue, as the legions were apparently being dispatched to prevent or deal with rebellion. A cavalry force was sent to hold down the Germanic and Belgic tribes. Troops under Publius Crassus were sent to Aquitania, and Quintus Titurius Sabinus took forces to Normandy. Caesar led the remaining four legions overland to meet up with his recently raised fleet near the mouth of the river Loire.

The Veneti held the upper hand for much of the campaign. Their ships were well-suited to the region, and when their hill forts were under siege, they could simply evacuate them by sea. The less sturdy Roman fleet was stuck in harbor for much of the campaign. Despite having the superior army and great siege equipment, the Romans were making little progress. Caesar realized that the campaign could not be won on land and halted the campaign until the seas calmed enough for the Roman vessels to be most useful.