Roman client kingdoms in Britain

The Roman client kingdoms in Britain were native British polities that maintained allied relationships with the Roman Empire and retained local rulers under Roman patronage. Alternatively, the Romans created or enlisted some client kingdoms when influence without direct rule was desirable. Client kingdoms were ruled by client kings. In Latin these kings were referred to as rex sociusque et amicus, which translates to "king, ally, and friend". The relationships between client kingdoms and Rome depended on the circumstances in each kingdom.

The beginnings of the system are found in Caesar's re-enthroning of Mandubracius as king of the Trinovantes, who had been dethroned by Cassivellaunus and then aided Caesar's second invasion of Britain in 54 BC. The system developed further during the following century, particularly under Augustus's influence, so that by the time of the Roman invasion in AD 43 several Roman client kingdoms had become established in the south of Britain. Client kingdoms were annexed when Rome reaffirmed its power in Britain or when client rulers were no longer able to govern their territories effectively.

These were also partially due to the expansion of the Catuvellauni under Cunobelinus in the southeast, and partly as a result of the invasion itself, and included Cogidubnus of the Regni, Prasutagus of the Iceni and Cartimandua of the Brigantes and, probably, of the Dobunni. The antecedents of the Regni, the Atrebates, had (in their Gallic and British forms) been client kingdoms of Rome since Caesar's first invasion in 55 BC. In the north of Britain, ongoing border struggles across the defensive walls led to the establishment of buffer states, including the Votadini in Northumberland.