[Sidenote: whose chariots harass his cavalry.]
During a great part of the march Cassivellaunus dogged the Roman column. Caesars object was to strike terror; and despoil the inhabitants of their chief source of wealth,--their flocks and herds. But Cassivellaunus soon taught him a lesson of caution. He succeeded in ascertaining what route the Romans intended to pursue, and sent messengers to warn the inhabitants to drive their cattle into the woods and to fly for refuge thither themselves. Knowing every inch of the country, and having the advantage of superior mobility, he would conceal his force in some wooded spot, and when he saw the Roman horsemen diverge from the column and ride forth to plunder, swoop down upon them and inflict heavy loss. Caesar was compelled to keep his cavalry, who were terrorized by these unforeseen attacks, in constant touch with the infantry; while the legions, whose powers of endurance were taxed to the uttermost, moved off the road from time to time, and burned and ravaged whatever they could reach.[1388]
Caesar had ascertained that the Thames, in that part of its course which formed the southern boundary of the territory of Cassivellaunus, was only fordable at one spot; and since the time of Camden it has generally been supposed that this was close to Halliford,--the only place, it is said, between Hurleyford, about two miles west of Great Marlow, and the sea, whose name preserves the memory of an ancient ford.[1389] Evidence, however, has lately been adduced which makes it more probable that Caesar was describing Brentford; for, though the name may only have denoted a ford over the Brent, in this part only of the lower Thames have piles been discovered in dredging operations which could reasonably be identified with the obstacles that threatened the passage of the Roman army.[1390] When the column descended into the valley, Caesar found that Cassivellaunus had anticipated him. The further bank was fenced by a row of sharp stakes, behind which were massed Cassivellaunus’s tribesmen; and Caesar learned from prisoners and deserters that similar stakes, concealed by the water, were planted in the bed of the river. He sent his cavalry behind cover to swim the stream close by; and at the right moment the column of infantry plunged into the water, and advanced to the attack. Caesar had calculated that the British levies would be distracted by the onset of the cavalry upon their flanks and rear; but the infantry were determined to have the credit for themselves. We may suppose that, while they were removing the stakes, the slingers and archers harassed the enemy.[1391] ‘The infantry,’ wrote Caesar, ‘advanced with such swiftness and dash, though they had only their heads above water, that the enemy, unable to withstand the combined onset of cavalry and infantry, abandoned the bank and fled.’[1392]