Cassivellaunus in the context of "Caesar's invasions of Britain"

The Roman client kingdoms in Britain were native tribes which chose to align themselves with the Roman Empire because they saw it as the best option for self-preservation or for protection from other hostile tribes. Alternatively, the Romans created (or enlisted) some client kingdoms when they felt 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 type of relationships between client kingdoms and Rome was reliant on the individual circumstances in each kingdom.

The beginnings of the system are to be 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 further developed in the following hundred years, particularly under Augustus's influence, so that by the time of the Roman invasion in 43 AD several Roman client kingdoms had become established in the south of Britain. Client kingdoms were annexed when Rome needed to reaffirm their power in Britain or when the client kings could not manage the kingdoms and surrounding areas any more.

The Cenimagni were a tribe of Iron Age Britain in the first century BCE. They are known only from a brief mention in the writings of Julius Caesar. It has been suggested that the name is a variant of Iceni with the Latin adjective magni, meaning "great". Others have suggested that they may have been one of the four tribes of Kent, represented in Caesar by references to the "four kings of that region" and in the archaeological record by distinct pottery assemblages.

During Julius Caesar's second invasion of Britain in 54 BCE, following Caesar's military success and restoration of King Mandubracius to power over the Trinovantes, opposition to the Romans coalesced around the figure of Cassivellaunus which led to divided loyalties among the Britons, as Caesar records. Emissaries of five British tribes, including the Cenimagni (the others being the Ancalites, the Segontiaci, the Bibroci and the Cassi), arrived at the Roman camp to treat for peace, and agreed to reveal details of Cassivellaunus' stronghold. Caesar besieged him there and brought him to terms. When Caesar left Britain he took hostages from the Britons, although which tribes were compelled to give any is not specified.