If the Belgæ had crossed with their whole army, they could have carried out their purpose; the Roman communications would have been broken, but the Romans could have gone ahead, and the Belgæ, outside of their own land, had no way of maintaining their supplies. The only thing to do was to surround the Roman camp from all sides and starve it out. Even with their superior numbers, which Cæsar gives as 306,000, this was a difficult operation, for the enveloping lines, owing to the country being traversed by two rivers, would have been large. In any case the Belgæ recognized that they could not keep the field long, and when they heard that Cæsar’s allies, the Ædui, were invading their country, they decided to withdraw, the confederated tribes engaging to help one another if Cæsar’s army invaded their territory. The retreat of the Belgæ was so unexpected that at first the Romans took it for a feint meant to provoke them to leave their camp. As soon as the news was well authenticated, the cavalry pursued the retreating barbarians, keeping up a series of irritating attacks. The Belgic strongholds surrendered soon after; only three tribes, the Nervii, the Viromandui, and the Atrebates, tried to strike a blow for Gallic freedom. They fell upon the Romans, while they were arranging to encamp in a woody country on the Sambre, and caused almost a panic. The allied troops fled in confusion, but the legionaries held their ground, getting themselves in line, and as they were far superior in numbers to the Nervii, they soon got the upper hand of them, although there was some sharp fighting and for a time two of the legions were hard pressed. It was part of the Roman general’s strategy not to face a superior force. This point is apparent in the previous campaigns, but, as a military writer, Cæsar had no scruples in manipulating his figures for popular consumption. When the Nervii made peace unconditionally, they represented themselves, according to Cæsar, as having only 500 men left out of an original 60,000 capable of bearing arms; a few years later they appear again in the Commentaries as having a considerable army. They also sent a contingent of 5000 to Alesia at the close of the Gallic war. Probably a just estimate of the fighting force of the Nervii would give them 30,000 men, because the whole population of the district could hardly have been more than 150,000 souls. They occupied a territory of four hundred square kilometers, and with the slight density of population in Gaul, they could not have numbered more than the figures given above. Even in the Italian peninsula, which was more thickly settled, there was altogether a population of not more than three and a half millions and a density of only twenty-five per square kilometer. The Roman legions who opposed the Nervii in this last fight numbered at least 40,000 men.