C. Julius Caesar, Gallic War

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 marshaled 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 ardor, 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.