The Nervii set up an ambush along the Sambre river, lying in wait for the Romans who arrived and started setting a camp. The Romans detected the Nervii, and the battle began with the Romans sending a light cavalry and infantry force across the river to keep the Nervii at bay while the main force fortified its camp. The Nervii easily repulsed the attack. In an uncharacteristic move for Caesar, he made a serious tactical error by not setting up an infantry screen to protect the entrenching force. The Nervii took ample advantage of this, and their entire force crossed the river at speed and caught the Romans off-guard and unprepared. As the battle began, two legions had not even arrived, whereas the Nervii had at least 60,000 fighters.