After the submission of the Ambiani, Cæsar left Amiens to proceed to the country of the Nervii; and after three days’ march on their territory, he arrived probably at Bavay (_Bagacum_), which is considered to have been their principal town. There he learnt by prisoners that he was no more than ten miles (fifteen kilomètres) distant from the Sambre, and that the enemy awaited him posted on the opposite bank of the river.[258] He thus found himself on the left bank, and the Nervii were assembled on the right bank.[259] (_See Plate 7._) In accordance with the informations he had received, Cæsar sent out a reconnoitring party of scouts and centurions, charged with the selection of a spot favourable for the establishment of a camp. A certain number of the Belgæ, who had recently submitted, and other Gauls, followed him, and accompanied him in his march. Some of them, as was known subsequently by the prisoners, having observed during the preceding days the usual order of march of the army, deserted during the night to the Nervii, and informed them that behind each of the legions there was a long column of baggage; that the legion which arrived first at the camp being separated by a great space from the others, it would be easy to attack the soldiers, still charged with their bundles (_sarcinæ_); that this legion once routed and its baggage captured, the others would not dare to offer any resistance. This plan of attack was the more readily embraced by the Belgæ, as the nature of the locality favoured its execution. The Nervii, in fact, always weak in cavalry (their whole force was composed of infantry), were accustomed, in order to impede more easily the cavalry of their neighbours, to notch and bend horizontally young trees, the numerous branches of which, interlaced and mingled with brambles and brushwood, formed thick hedges, a veritable wall which nothing could pass through, impenetrable even to the eye.[260] As this kind of obstacle was very embarrassing to the march of the Roman army, the Nervii resolved to hide themselves in the woods which then covered the heights of Haumont, to watch there the moment when it would debouch on the opposite heights of the Sambre, to wait till they perceived the file of baggage, and then immediately to rush upon the troops which preceded.[261] (_See Plate 10._) [Sidenote: Battle on the Sambre.]