In the course of his Gallic Wars, Julius Caesar invaded Britain twice: in 55 and 54 BC. On the first occasion, Caesar took with him only two legions, and achieved little beyond a landing on the coast of Kent. The second invasion was more substantial, consisting of 800 ships, five legions and 2,000 cavalry. The Celtic Britons did not initially contest Caesar's landing, waiting instead until he began to move inland. Caesar eventually penetrated into Middlesex and crossed the Thames, forcing the British warlord Cassivellaunus to pay tribute to Rome and setting up Mandubracius of the Trinovantes as a client king. The Romans then returned to Gaul without conquering any territory.
Caesar included accounts of both invasions in his Commentarii de Bello Gallico, which contains the earliest surviving significant eyewitness descriptions of the island's people, culture and geography. This is effectively the start of the written history, or at least the protohistory, of Great Britain.
Britain before Caesar
Britain had long been known to the classical world as a source of tin. The coastline had been explored by the Greek geographer Pytheas in the 4th century BC, and may have been explored even earlier, in the 5th century, by the Carthaginian sailor Himilco. But to many Romans, the island, lying as it did beyond the Ocean at what was to them the edge of the known world, was a land of great mystery. Some Roman writers even insisted that it did not exist and dismissed reports of Pytheas's voyage as a hoax.
Britain during the reign of Julius Caesar had an Iron Age culture, with an estimated population of between one and four million. Archaeological research shows that its economy was broadly divided into lowland and highland zones. In the lowland southeast, large areas of fertile soil made possible extensive arable farming, and communication developed along trackways, such as the Icknield Way, the Pilgrims' Way and the Jurassic Way, and navigable rivers such as the Thames. In the highlands, north of the line between Gloucester and Lincoln, arable land was available only in isolated pockets, so pastoralism, supported by garden cultivation, was more common than settled farming, and communication was more difficult. Settlements were generally built on high ground and fortified, but in the southeast, oppida (fortified towns) had begun to be established on lower ground, often at river crossings, suggesting that trade was becoming more important. Commercial contact between Britain and the continent had increased since the Roman conquest of Transalpine Gaul in 124 BC, and Italian wine was being imported via the Armorican peninsula, much of it arriving at Hengistbury Head in Dorset.
Caesar's written account of Britain says that the Belgae of northeastern Gaul had previously conducted raids on Britain, establishing settlements in some of its coastal areas, and that within living memory Diviciacus, king of the Suessiones, had held power in Britain as well as in Gaul. British coinage from this period shows a complicated pattern of intrusion. The earliest Gallo-Belgic Celtic coins that have been found in Britain date to before 100 BC, perhaps as early as 150 BC, were struck in Gaul and have been found mainly in Kent. Later coins of a similar type were struck in Britain and are found all along the south coast as far west as Dorset. It appears that Belgic power was concentrated on the southeastern coast, although their influence spread further west and inland, perhaps through chieftains establishing political control over the native population.
Motivation
Caesar had been conquering Gaul since 58 BC and in 56 BC he took most of northwest Gaul after defeating the Veneti in the naval Battle of Morbihan.
Caesar's pretext for the invasion was that "in almost all the wars with the Gauls succours had been furnished to our enemy from that country" with fugitives from among the Gallic Belgae fleeing to Belgic settlements in Britain, and the Veneti of Armorica, who controlled seaborne trade to the island, calling in aid from their British allies to fight for them against Caesar in 56 BC. Strabo says that the Venetic rebellion in 56 BC had been intended to prevent Caesar from travelling to Britain and disrupting their commercial activity, suggesting that the possibility of a British expedition had already been considered by then.
It may also have been a cover for investigating Britain's mineral resources and economic potential: afterwards, Cicero refers to the disappointing discovery that there was no gold or silver in the island; and Suetonius reports that Caesar was said to have gone to Britain in search of pearls.
However, it may have been an excuse to gain stature in the eyes of the Roman people, due to Pompey and Crassus' consulship. On the one hand, they were Caesar's political allies, and Crassus's son had fought under him the year before. But they were also his rivals, and had formidable reputations (Pompey was a great general, and Crassus was fabulously wealthy). Since the consuls could easily sway and buy public opinion, Caesar needed to stay in the public eye. His solution was to cross two water bodies no Roman army had attempted before: the Rhine and the English Channel.
First invasion (55 BC)
Planning and reconnaissance
Caesar summoned merchants who traded with the island, but they were unable or unwilling to give him any useful information about the inhabitants and their military tactics, or about harbours he could use, presumably not wanting to lose their monopoly on cross-channel trade. He sent a tribune, Gaius Volusenus, to scout the coast in a single warship. He probably examined the Kent coast between Hythe and Sandwich, but was unable to land, since he "did not dare leave his ship and entrust himself to the barbarians", and after five days returned to give Caesar what intelligence he had managed to gather.
By then, ambassadors from some of the British states, warned by merchants of the impending invasion, had arrived promising their submission. Caesar sent them back, along with his ally Commius, king of the Belgae Atrebates, to use their influence to win over as many other states as possible.
He gathered a fleet consisting of eighty transport ships, sufficient to carry two legions (Legio VII and Legio X), and an unknown number of warships under a quaestor, at an unnamed port in the territory of the Morini, almost certainly Portus Itius (Saint-Omer). Another eighteen transports of cavalry were to sail from a different port, probably Ambleteuse. These ships may have been triremes or biremes, or may have been adapted from Venetic designs Caesar had seen previously, or may even have been requisitioned from the Veneti and other coastal tribes.
In late summer 55 BC, even though it was late in the campaigning season, Caesar decided to embark for Britain.
Landing
Caesar moved quickly, leaving a garrison at the port and set out "at the third watch" past midnight on the 23 August with the legions so that they would arrive at dawn, leaving the cavalry to march to their ships, embark, and join him as soon as possible. Though Caesar could not have known, many historians consider that Caesar's separation of the cavalry from the legions was possibly a tactical mistake due to the infantry lacking baggage or heavy siege gear. Alternatively, it could also suggest that the invasion was not intended for complete conquest.
During Caesar's initial attempts at making a landing, the massed forces of the Britons gathered on the overlooking hills dissuaded him. After waiting at anchor for nine hours until around 3 PM, waiting both for his supply ships from the second port to come up and also convening a war council, Caesar ordered the fleet to sail along the coast to an open beach.
The massive Briton force, including cavalry and chariots kept pace with the fleet, making the legions hesitant to attempt a landing. In addition to lacking cavalry and heavy equipment, the loaded Roman ships were too low in the water to go close inshore and the troops had to disembark in deeper water, making them vulnerable to enemy attack while in the shallows. To break the stalemate, it was recorded that the legion's standard bearer jumped into the sea and waded to shore. Due to the fact that having the legion's standard fall in combat was a great humiliation, and the other men were also forced to disembark. After some delay, a battle line was eventually formed, and the Britons withdrew. The cavalry auxiliaries were unable to make the crossing despite several attempts, preventing Caesar from chasing down the Britons.
Skirmishes
The Romans established a camp and received ambassadors and had Commius, who had been arrested as soon as he had arrived in Britain, returned to them. Caesar claims he was negotiating from a position of strength and that the British leaders, blaming their attacks on him on the common people, were in only four days awed into giving hostages, some immediately, some as soon as they could be brought from inland, and disbanding their army. However, after his cavalry had come within sight of the beachhead but then been scattered and turned back to Gaul by storms, and with food running short, Caesar was taken by surprise by high British tides and a storm. His beached warships filled with water, and his transports, riding at anchor, were driven against each other. Some ships were wrecked, and many others were rendered unseaworthy by the loss of rigging or other vital equipment, threatening the return journey.
Realising this and hoping to keep Caesar in Britain over the winter and thus starve him into submission, the Britons renewed the attack, ambushing one of the legions as it foraged near the Roman camp. The foraging party was relieved by the remainder of the Roman force and the Britons were again driven off, only to regroup after several days of storms with a larger force to attack the Roman camp. This attack was driven off fully, in a bloody rout, with improvised cavalry that Commius had gathered from pro-Roman Britons and a Roman scorched earth policy.
The campaigning season was now nearly over, and the legions were in no condition to winter on the coast of Kent. Caesar withdrew back across the Channel with as many of the ships as could be repaired with flotsam from the wrecked ships.
Conclusion
Caesar again avoided a set-back. Taking an understrength army with few provisions to Britain was a decision that carried a high risk from a strategic view-point, which easily could have led to Caesar's defeat, yet he survived. While he had achieved no significant gains in Britain, he had accomplished a notable feat simply by landing there. It was a propaganda victory as well, which was chronicled in Caesar's ongoing Commentarii de Bello Gallico. The writings in the Commentarii fed Rome a steady update of Caesar's exploits (with his own personal spin on events). Caesar's goal of prestige and publicity succeeded: upon his return to Rome, he was hailed as a hero and given an unprecedented 20-day thanksgiving.
Second invasion (54 BC)
Preparation
Caesar's approach in the winter of 55–54 BC towards the invasion in 54 BC was far more comprehensive and successful than his initial expedition. New ships had been built over the winter, using experience of Venetic shipbuilding technology being broader and lower for easier beaching, and Caesar now took 800 ships, five legions (instead of two) and 2,000 cavalry. He left the rest of his army in Gaul to keep order. Caesar took with him a good number of Gallic chiefs whom he considered untrustworthy so he could keep an eye on them.
This time he named Portus Itius as the departure point.
Crossing
Titus Labienus was left at Portus Itius to oversee regular food transports from there to the British beachhead. The military ships were joined by a flotilla of trading ships captained by Romans and provincials from across the empire, and local Gauls, hoping to cash in on the trading opportunities. It seems more likely that the figure Caesar quotes for the fleet (800 ships) include these traders and the troop-transports, rather than the troop-transports alone.
The Roman fleet sailed from France in the evening so that the army could land in daylight. They hoped to use the wind to help cross the Channel, but by midnight the wind dropped and the channel tide carried them too far northeast and at sunrise they saw Britain in the distance on their left. They managed to row and use the reversing tide to arrive at the place identified as the best landing-place the previous year.