XVIII.--Caesar, discovering their design, leads his army into the territories of Cassivellaunus to the river Thames; which river can be forded in one place only, and that with difficulty. When he had arrived there, he perceives that numerous forces of the enemy were marshalled on the other bank of the river; the bank also was defended by sharp stakes fixed in front, and stakes of the same kind fixed under the water were covered by the river. These things being discovered from [some] prisoners and deserters, Caesar, sending forward the cavalry, ordered the legions to follow them immediately. But the soldiers advanced with such speed and such ardour, though they stood above the water by their heads only, that the enemy could not sustain the attack of the legions and of the horse, and quitted the banks, and committed themselves to flight. XIX.--Cassivellaunus, as we have stated above, all hope [rising out] of battle being laid aside, the greater part of his forces being dismissed, and about 4000 charioteers only being left, used to observe our marches and retire a little from the road, and conceal himself in intricate and woody places, and in those neighbourhoods in which he had discovered we were about to march, he used to drive the cattle and the inhabitants from the fields into the woods; and, when our cavalry, for the sake of plundering and ravaging the more freely, scattered themselves among the fields, he used to send out charioteers from the woods by all the well-known roads and paths, and, to the great danger of our horse, engage with them; and this source of fear hindered them from straggling very extensively. The result was that Caesar did not allow excursions to be made to a great distance from the main body of the legions, and ordered that damage should be done to the enemy in ravaging their lands and kindling fires only so far as the legionary soldiers could, by their own exertion and marching, accomplish it. XX.--In the meantime, the Trinobantes, almost the most powerful state of those parts, from which the young man Mandubratius embracing the protection of Caesar had come to the continent of Gaul to [meet] him (whose father, Imanuentius, had possessed the sovereignty in that state, and had been killed by Cassivellaunus; he himself had escaped death by flight), send ambassadors to Caesar, and promise that they will surrender themselves to him and perform his commands; they entreat him to protect Mandubratius from the violence of Cassivellaunus, and send to their state some one to preside over it, and possess the government. Caesar demands forty hostages from them, and corn for his army, and sends Mandubratius to them. They speedily performed the things demanded, and sent hostages to the number appointed, and the corn.