The Roman emperor Claudius ordered the invasion of Britain in 43 AD, with the target of the campaign a fortified Iron Age settlement known then as Camulodunum, near what is now modern Colchester. Some twenty years later the Iceni were ruled by Prasutagus, and his kingdom was considered a civitates peregrinae, where non-citizens were allowed an amount of self-govenment, provided they were obedient. ""The Britons themselves submit to the levy, the tribute and the other charges of Empire with cheerful readiness," Tacitus wrote, "provided that there is no abuse. That they bitterly resent: for they are broken in to obedience, not to slavery". When he died, he left his kingdom jointly to his wife, Boudica, his two daughters and the Roman Emperor. However, unlike the native Britons, Rome did not recognize female inheritance and annexed the kingdom. What happened next was seizure and oppression; the Romans were conquerors, and the governor Suetonius Paullinus treated the Britons as a defeated enemy:
- "Kingdom and household alike were plundered like prizes of war, the one by Roman officers, the other by Roman slaves. As a beginning, his widow Boudicca was flogged and their daughters raped. The Icenian chiefs were deprived of their hereditary estates as if the Romans had been given the whole country. The king's own relatives were treated like slaves."
Enraged at her treatment and the rape of her daughters, Boudica began her vengeance. While the Roman governor was distracted by a campaign against rebels on the island of Anglesey in north Wales, Boudica led a revolt of the Iceni along with other native tribes. The Roman city of Colchester (Camulodunum) was razed so thoroughly that the thick layer of ash left is still clearly visible today. The Roman IX Legion was routed and between 70,000 and 80,000 civilians were said to have been killed. The Roman historian Cassius Dio left an account of the campaign, leaving a brief description as to Boudica's appearance:
- "...a terrible disaster occurred in Britain. Two cities were sacked, eighty thousand of the Romans and of their allies perished, and the island was lost to Rome. Moreover, all this ruin was brought upon the Romans by a woman, a fact which in itself caused them the greatest shame....But the person who was chiefly instrumental in rousing the natives and persuading them to fight the Romans, the person who was thought worthy to be their leader and who directed the conduct of the entire war, was Buduica, a Briton woman of the royal family and possessed of greater intelligence than often belongs to women....In stature she was very tall, in appearance most terrifying, in the glance of her eye most fierce, and her voice was harsh; a great mass of the tawniest hair fell to her hips; around her neck was a large golden necklace; and she wore a tunic of divers colours over which a thick mantle was fastened with a brooch. This was her invariable attire."
Hearing of the rebellion while putting down the Druids on the Isle of Anglesey, Paullinus rushed to Londinium; among his ranks were the XIV Legion and XX Legion. Furthermore he sent his cavalry ahead with orders orders for II Legion to meet him there, only to discover upon arrival that with the defeat of IX Legion Londinium had to be abandoned for want of troops to defend it. Both Londinium and St Albans (Verulamium) were burned, and whoever was left behind were slaughtered.
After Boudica's initial successes, Paullinus rallied his forces against the revolt and engaged the rebels at Watling Street, near Mancetter, on ground of his choosing, and near an established Roman camp. Despite overwhelming numerical supremacy (Dio estimated 230,000) Boudica's disorganised army was defeated by the Roman's superior tactics. It is said that following the defeat she committed suicide by taking poison.