In response, the other Belgae and Celtic tribes had attacked Bibrax (the oppidum of the Remi, situated near the Aisne River). Caesar countered by defending the oppidum and winning an action at the Aisne. In the face of this and because of shortage of provisions, the union collapsed and tribal armies retreated to their own lands, with the agreed future intention of coming to the support of any tribe invaded by the Romans. Caesar continued his advance and tribes surrendered one by one. However, four tribes, the Nervii, the Atrebates, the Aduatuci, and the Viromandui refused to submit.