Julius Caesar describes the armed resistance put up by the British after his landing in 54BCE.
Gaius Julius Caesar invaded Britain twice – in 55 and 54BCE.
The first of these incursions was little more than a reconnaisance mission, which established a beachhead on the Kent coast but went no further. The second proved more enduring, resulting in the defeat of Cassivellaunus, and his replacement with the more Roman-friendly Mandubracius.
But having won a military victory, the Romans then left Britain. They would not return until 43AD, when Britain was incorporated into the Roman Empire. It would remain a colony of Rome until the last legions were withdrawn in 410AD.
This description of the invasion of Britain was given by Julius Caesar in his account of the Gallic wars
Julius Caesar on Britain, 54BCE
The horse and charioteers of the enemy contended vigorously in a skirmish with our cavalry on the march; yet so that our men were conquerors in all parts, and drove them to their woods and hills; but, having slain a great many, they pursued too eagerly, and lost some of their men. But the enemy, after some time had elapsed, when our men were off their guard, and occupied in the fortification of the camp, rushed out of the woods, and making an attack upon those who were placed on duty before the camp, fought in a determined manner; and two cohorts being sent by Caesar to their relief, and these severally the first of two legions, when these had taken up their position at a very small distance from each other, as our men were disconcerted by the unusual mode of battle, the enemy broke through the middle of them most courageously, and retreated thence in safety. That day, Q. Laberius Durus, a tribune of the soldiers, was slain. The enemy, since more cohorts were sent against them, were repulsed.
In the whole of this method of fighting since the engagement took place under the eyes of all and before the camp, it was perceived that our men, on account of the weight of their arms, inasmuch as they could neither pursue [the enemy when] retreating, nor dare quit their standards, were little suited to this kind of enemy; that the horse also fought with great danger, because they [the Britons] generally retreated even designedly, and, when they had drawn off our men a short distance from the legions, leaped from their chariots and fought on foot in unequal [and to them advantageous] battle. But the system of cavalry engagement is wont to produce equal danger, and indeed the same, both to those who retreat and to those who pursue. To this was added, that they never fought in close order, but in small parties and at great distances, and had detachments placed [in different parts], and then the one relieved the other, and the vigorous and fresh succeeded the wearied.
The following day the enemy halted on the hills, a distance from our camp, and presented themselves in small parties, and began to challenge our horse to battle with less spirit than the day before. But at noon, when Caesar had sent three legions, and all the cavalry, with C. Trebonius, the lieutenant, for the purpose of foraging, they flew upon the foragers suddenly from all quarters, so that they did not keep off [even] from the standards and the legions. Our men making an attack on them vigorously, repulsed them; nor did they cease to pursue them until the horse, relying on relief, as they saw the legions behind them, drove the enemy precipitately before them, and slaying a great number of them, did not give them the opportunity either of rallying, or halting, or leaping from their chariots. Immediately after this retreat, the auxiliaries who had assembled from all sides, departed; nor after that time did the enemy ever engage with us in very large numbers.
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.
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 4,000 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 neighborhoods 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.
In the mean time, 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 embassadors 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.
The Trinobantes being protected and secured from any violence of the soldiers, the Cenimagni, the Segontiaci, the Ancalites, the Bibroci, and the Cassi, sending embassies, surrendered themselves to Caesar. From them he learns that the capital town of Cassivellaunus was not far from that place, and was defended by woods and morasses, and a very large number of men and of cattle had been collected in it. (Now the Britons, when they have fortified the intricate woods, in which they are wont to assemble for the purpose of avoiding the incursion of an enemy, with an intrenchment and a rampart, call them a town.) Thither he proceeds with his legions: he finds the place admirably fortified by nature and art; he, however, undertakes to attack it in two directions. The enemy, having remained only a short time, did not sustain the attack of our soldiers, and hurried away on the other side of the town. A great amount of cattle was found there, and many of the enemy were taken and slain in their flight.
While these things are going forward in those places, Cassivellaunus sends messengers into Kent, which, we have observed above, is on the sea, over which districts four several kings reigned, Cingetorix, Carvilius, Taximagulus and Segonax, and commands them to collect all their forces, and unexpectedly assail and storm the naval camp. When they had come to the camp, our men, after making a sally, slaying many of their men, and also capturing a distinguished leader named Lugotorix, brought back their own men in safety. Cassivellaunus, when this battle was reported to him as so many losses had been sustained, and his territories laid waste, being alarmed most of all by the desertion of the states, sends embassadors to Caesar [to treat] about a surrender through the mediation of Commius the Atrebatian. Caesar, since he had determined to pass the winter on the continent, on account of the sudden revolts of Gaul, and as much of the summer did not remain, and he perceived that even that could be easily protracted, demands hostages, and prescribes what tribute Britain should pay each year to the Roman people; he forbids and commands Cassivellaunus that he wage not war against Mandubratius or the Trinobantes.
When he had received the hostages, he leads back the army to the sea, and finds the ships repaired. After launching these, because he had a large number of prisoners, and some of the ships had been lost in the storm, he determines to convey back his army at two embarkations. And it so happened, that out of so large a number of ships, in so many voyages, neither in this nor in the previous year was any ship missing which conveyed soldiers; but very few out of those which were sent back to him from the continent empty, as the soldiers of the former convoy had been disembarked, and out of those (sixty in number) which Labienus had taken care to have built, reached their destination; almost all the rest were driven back, and when Caesar had waited for them for some time in vain, lest he should be debarred from a voyage by the season of the year, inasmuch as the equinox was at hand, he of necessity stowed his soldiers the more closely, and, a very great calm coming on, after he had weighed anchor at the beginning of the second watch, he reached land at break of day and brought in all the ships in safety.
Source: The Gallic Wars V, by Julius Caesar, translated by W. A. McDevitte and W. S. Bohn