In the near century that followed Caesar's invasions of Britain (55 and 54 BC), the political climate of the Celtic tribes that maintained loyal relationships with Rome slowly deteriorated. While some call Caesars invasions a failure, the fact that they resulted in a century of tribute, profitable trade, and subservient political attitudes from the tribes to Rome must account for something. However, the more removed Caesar was from the memory of the Celtic tribes, the less stable the relationship between Rome and Britain remained. Rome could ill afford a break in the trading relationship that provided invaluable metals such as silver, tin and iron and immense profits for other 'commercial' industries such as wine making and pottery. An abundance of lead ore had also recently been discovered and it was an important material to Romans in many industries.
An invasion had long been planned, as far back as the botched attempts of Caligula just a few years earlier, to deal with apparent affronts to Roman allied tribes. These proved to be too costly, dangerous or perhaps just truly unnecessary for the time, however, and the British political situation would continue to destabilize (from a Roman perspective). By the time Claudius came to power, not only was King Verica (descendent of Caesar's ally Commius) of the allied Atrebates calling to Rome for help against neighboring tribes, but Claudius was conveniently in need of military glory to secure the loyalty of the legions. Verica's pleas provided a convenient excuse to not only gain this glory, but to distract the Roman Senate from political differences of opinion. Making matters even more persuasive, the Rhine and Danube borders of Germania were largely quiet, and the empire was largely at peace. The invasion could take place without stripping troops from other vital areas.
In AD 43, Aulus Plautius, former governor of Pannonia and a manned deemed trustworthy by Claudius, was selected to lead the overall campaign. 4 known legions were gathered, and put through rigorous drilling, in order to weed out those who were unfit. Legio II Augusta, IX Hispana, XIV Gemina and XX Valeria Victrix were all to take part, with II Augusta under the command of the future emperor Vespasian. The legions totaled 20,000 active legionaries, with an additional 20,000 to 30,000 in auxilia forces. The initial channel crossing was set to take place early in the campaign year, but dismay among the troops caused considerable problems. Roman infantry had long been known to have misgivings about naval crossings, but sailing to the mysterious island of Britain, where even the great Caesar had faced considerable problems, was another matter entirely. The situation was eventually resolved through the intervention of Claudius' freedman advisor Narcissus, but the affair was delayed for some time.
Nevertheless, the bulk of the force seems to have landed near Rutupiae, or modern Richborough. Though some evidence exists of additional landing places, there is no question that Richborough developed into the main entry port in provincial Britain with it acting as a sort of ancient version to Ellis Island in New York harbor. Regardless, the initial landings took place, mostly unopposed. Tribal resistance under Caratacus (also Caractacus) king of the Catuvellauni, and his brother Togodumnus, was slow to react and the Romans were able to gather in force. The Britons had little choice but to allow the invaders into their lands, until such time as an appropriate force could be gathered. Tribal divisions and diverse loyalties made resistance a trying affair. While many opposed Roman occupation, leadership squabbles certainly made a unified front difficult to achieve. Other less influential tribes welcomed Roman arrival and supported their cause, if only with the belief that doing so would increase their own power. The socio-political environment of tribal Britain made the Roman strategy easy to understand. Aulus Plautius would make his campaign one of divide and conquer, much like Caesar had done in Gaul a century earlier.
The first major battle of the campaign (and perhaps one of the most important in British history) took place on the shores of the River Medway (though some contend that it may have been the Arun). Caratacus and Togodumnus gathered their considerable forces but were unable to resist the superior tactical and strategic abilities of the Romans. In a terrific battle that reportedly lasted two days, where Germanic Batavi auxilia, who swam the river to surprise the British rear guard, played a vital role, the British forces were eventually outmanned and forced to retreat. Caratacus was forced back to the Thames, and Togodumnus was eventually killed in ensuing actions. Apparantly the popularity of Togodumnus was such among the British warriors however; that they had a new resolve to resist the invaders and Plautius was 'forced' to dig in. This provided a convenient excuse for Platius to send to Rome and ask Claudius to come to Britain in person to conduct the campaign.
Though a couple months had passed while Claudius made his own preparations, Plautius certainly was in a position where he was forced to await the Emperor's arrival, before a final assault could be launched. There may be some truth that the Romans were bogged down and lacked morale, of which Claudius' arrival would certainly help, but the idea that experienced Roman generals needed his military advice in any way is completely ludicrous. Regardless, Caludius eventually arrived, bringing up to 38 war elephants with him, as well as heavy artillery and (some evidence suggests) detachments of the 8th Legion. Claudius took command, and with this superior strength overwhelmed the resistance and Caratacus was forced to flee into modern Wales with his army. In a period of only 16 days, Claudius captured Camulodunum, where the first Roman capital was established, and received the surrender of up to 11 British tribes. The Emperor returned to Rome where he celebrated a grand triumph, received the title Britannicus, and left his generals in Britain to complete the conquest.
Though southern Britain would be largely subjugated relatively quickly, Roman control would spread slowly over the next 20 years. Men such as Vespasian built reputations as excellent generals and spread Roman hegemony in numerous and sometimes difficult campaigns. While Claudius had already celebrated his triumph in Rome, the exhausting work of Romanizing Britain was only just beginning. While it would eventually become an ideal province (especially in the south), a great deal of trouble lay ahead. This trouble would culminate in the revolt of Boudicca in the early 60's AD, and Britain wouldn't fall to complete Roman rule until the arrival of Gnaeus Julius Agricola some 20 years later.