I think that the neighbouring Catuvellauni may have resented the British Atrebates. This is only speculation, but why would the Catuvellauni upset the way of things and invade areas of Rome’s ally, unless they had reason to resent and underestimate the real reach of the Roman Empire. Perhaps they had the memory of the British Atrebates carving out territory for themselves back in 55-54 BC. Maybe historical resentment still flourished and two brother warrior chieftains decided to reclaim territory from mainland European Celts. They may have also believed that Rome might have happily traded with Catuvellauni policed Britain instead of British Atrebates having special treatment. If this was so, then the two brothers, Togodumus and Caractacus were wrong.
The expansion of Catuvellauni territory disturbed Rome and Emperor Claudius decided to take matters in hand. He ordered a full-scale Roman invasion of Britain. Not an expedition to establish client kingdoms, but a landing on British soil to establish a permanent presence in Britain. Contrary to popular belief, a number of the south-east British kingdoms were helpful to the Romans and the Catuvellauni bit off more than they could cope with.
The Roman Emperor Claudius sent Aulus Plautius and four legions into Britain. The Emperor himself also came to Britain during the campaign. The legions came up against guerrilla tactics but were able to move deep into the island of Britain. They won two major conflicts at the river Medway and the river Thames. At one of these battles, Togodumus was killed but Caractacus was forced to flee while the Catuvellauni kingdom fell to direct Roman rule. Emperor Claudius watched his victorious Legions march into Camulodunum – today's Colchester, Essex, England. It is worth pointing out that Camulodunum is regarded as being in the Trinovante kingdom during Boadicea’s revolt in 60 AD. So maybe the Trinovante, bordering Catuvellauni’s east may have welcomed the Romans as allies or they may also have been overrun. Trinovantes certainly went to Rome in the future coming rebellion of the Iceni Queen during Emperor Nero’s reign.
Caractacus fled west and remained at large encouraging British kingdoms like the Silures and Ordovices to fight the Roman invaders. These nations were in today’s Wales. Finally in 50 AD, some eight years after the fall of the Catuvellauni territory, Caractacus tried to take on the Roman army again. This was at the Battle of Cear Caradoc.
The old Roman commander, Aulus Plautius had been relieved of the governorship of Britain in 47 AD. The new Roman commander was called Publius Ostorius Scapula and he was confronted by Caractacus and an army of mainly Ordovices and some Silures warriors at Cear Caradoc Hill. They had built an encampment on the summit with stone ramparts and a river below, before the Roman army. The Celtic British warriors had the geographical advantage but the Romans crossed the river with a hail of missiles falling from the sky. They fashioned their tortoise style formations using their shields as an armoured wall. Front, sides, back and top were covered by their shields as they advanced up towards the British ramparts. They overran them and drove the British off, inflicting another major defeat that left Rome in control of all southern Britain. Caratacus, once again, fled leaving his wife and daughter captured by the Roman army.
This time, he made the most monumental of all his mistakes. He fled north to the Brigantes, hoping to find sanctuary in Queen Cartimunda’s client kingdom that was at peace with Rome. Caractacus was bound in chains and taken before the Romans. He was then transported to Rome and paraded before the Senate and Emperor Claudius to be sentenced to death. However, he made a speech that captured the imagination of the Senate and the Emperor and was spared execution. Instead, he was taken to a place of seclusion and allowed to live out his days somewhere in Italy. How long he lived, is not known, but he evaded execution.