From A History of Wales by John Davies, London, 2007, p. 26:
"Cunobelinus died about AD 40 and his kingdom was inherited by his sons, Caratacus and Togodumnus. Their brother, Amminius, had been exiled by Cunobelinus, and he appealed to Rome to help him gain a share of his father's territories. Amminius's appeal, along with the complaints of the tribes which had suffered from the attacks of the Belgae, provided the Romans with an excuse to invade the island, although their real motive was their desire to seize the fertile lowlands...
"In May AD 43, Aulus Plautius sailed across the Channel with four legions and a host of auxiliary soldiers--forty thousand men in all. Within three months, it was considered that Rome's hold upon south-eastern Britain was secure enough to allow the emperor Claudius, the most inoffensive member of the complex Julio-Claudian family, to visit the new province and to make a ceremonial entry into Camulodunum (Colchester), the capital of the Catuvellauni, on an elephant..."