The invasion of Britain was likely planned as early as 57 BC, and certainly by 56 BC. Aid by British Celts against Roman efforts in Gaul, gave Caesar the excuse he needed to justify the undertaking, but his motives were certainly far more personal and political. Much like his crossing of the Rhine into Germania, Caesar certainly wanted to be the first Roman to gain the prestige of crossing to Britain, the farthest reach of the known ancient world. The great mineral wealth of Britain, metals such as silver, iron and tin also were a likely motivation, and in 55 BC, an expedition was finally practical.
In late August of 55 BC, with the VII and X legions approximating 10,000 men, Caesar set sail from Portus Itius (modern Boulogne) reaching the British coast off of Dover overnight. Caesar had already been in lengthy discussion with various merchants and other Celts with knowledge of Britain, and the Britons were well aware of the coming expedition. Additionally, Caesar's ally Commius, chief of the Gallic Atrebates, had already gone to Britain to negotiate peace for the Romans and was taken prisoner by the Britons. Upon the initial arrival, Celtic warriors lined the cliffs of Dover, making a landing impossible, and the fleet was forced farther up the coast. Though followed by the Cantii, who had every intention of blocking the Roman landing, the Romans landed on a pebbly flat shore near Deal.
Caesar's initial planning, and in fact the entire first expedition, seems to have been fraught with uncharacteristic tactical errors. He first chose to land at low tide, forcing his ships to anchor up to 600 feet off shore. His men were then forced to wade that distance under heavy fire from British missiles. The landing was in peril from the beginning, with the men hesitant to rush the shore. Only the intervention of the 10th legion's standard bearer inspired the men. According to Caesar he began to rush the shore calling out, "Leap forth, soldiers, unless you wish to betray your standard to the enemy. I, at any rate, shall have performed my duty to my country and my general."
Initially, the British chariots caused considerable problems for the legionaries, and Caesar was impressed by the Celtic method of warfare. In his commentaries, he wrote, "Their manner of fighting from chariots is as follows : first they drive in all directions and hurl javelins, and so by the mere terror that the teams inspire and by the noise of the wheels they generally throw the ranks of soldiers into confusion. When they have worked their way in between the troops, they leap down from the chariots and fight on foot. Meanwhile their charioteers retire gradually from the battle, and place chariots in such a fashion that, if the warriors are had pressed by the enemy, they have a ready means of retreat to their own side "
The Romans eventually got the upper hand, however, and drove the Britons back. Over the next few days, many tribes began negotiating peace and surrendering to Caesar, but failure to provide hostages as was the custom, kept Caesar suspicious. On the fourth day after his arrival in Britain, his much needed cavalry arrived, but a terrible storm not only forced them back to Gaul. Worse still, the storm and changes in the tides damaged the bulk of Caesar's anchored fleet. The Romans had brought little food supply with them, and now faced the danger of being unable to return to Gaul. With only 2 legions, it seems likely from the ill-prepared nature of the expedition, that it certainly wasn't meant to be a full invasion. Caesar probably expected very little resistance among the locals, and this first trip was very likely only intended as a short show of strength to impress the Britons and arrange for terms. Regardless, the Romans were in serious trouble and had to begin foraging for supplies.
At this point, it became clear to the Celtic chiefs that if they had an opportunity to send the Romans back across the channel. With one legion busily working on salvaging and repairing the fleet, the second was sent out to reap corn in local fields. While foraging, the British ambushed the legion, and their chariots wreaked havoc. The Romans were simply unable to stand and fight in the disciplined style that they were trained for. Caesar claims that his presence kept the battle from a rout, and he was able to withdraw back to the beach and his other legion. A stroke of luck allowed the Romans time to regroup, when 3 days of rain prevented any more attacks. However, it also allowed the Britons time to share the news of its initial victory, thereby recruiting more warriors for the coming battle.
Caesar was in serious jeopardy of losing his first major encounter on foreign soil. Retreat was not an option due to weather, and Caesar's dignity. When the weather broke, a large Celtic army moved towards the Roman camp, and Caesar prepared in the Roman style. This time, though, he went on the offensive rather than allow his men to be intimidated by the British chariots. The battle turned out to be a rather short affair, ending in victory for the Romans. They pursued the fleeing enemy around the countryside burning and surrounding as they went, eventually forcing the local tribes to sue for peace. In order to save face, Caesar demanded double the number of hostages originally asked for, but promised to leave for Gaul as soon as able. The Britons agreed, though only a few hostages were actually sent, but since the fleet was ready Caesar hastily crossed back across the channel.
This first expedition was certainly no great Roman victory, and can really be considered a defeat at the hands of the British tribes. Though he escaped mostly unharmed, Caesar's pride and dignity were surely damaged. His report back to Rome for this campaign year was impressive. Having crossed the Rhine to Germania and across the sea to Britain, Caesar overcame any fault for being so poorly prepared. Another 20 days of thanksgiving were granted to him for his deeds in 55 BC, but when he returned to Cisalpine Gaul for winter administration, Britain was most definitely still on his mind.