The Cassi are one of five tribes encountered by Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

  during his second expedition

Caesar's invasions of Britain

In his Gallic Wars, Julius Caesar invaded Britain twice, in 55 and 54 BC. The first invasion, made late in summer, was either intended as a full invasion or a reconnaissance-in-force expedition...

 to Britain in 55 BC when he crossed the Thames at Kew, and who became his allies after the Trinovantes

Trinovantes

The Trinovantes or Trinobantes were one of the tribes of pre-Roman Britain. Their territory was on the north side of the Thames estuary in current Essex and Suffolk, and included lands now located in Greater London. They were bordered to the north by the Iceni, and to the west by the Catuvellauni...

 joined him. The archaeologists Graham Webster and Barry Cunliffe both agree that nothing more is known about them, but it has been suggested that between Caesar's second invasion and the invasion of Claudius in AD 43

Roman conquest of Britain

The Roman conquest of Britain was a gradual process, beginning effectively in AD 43 under Emperor Claudius, whose general Aulus Plautius served as first governor of Britannia. Great Britain had already frequently been the target of invasions, planned and actual, by forces of the Roman Republic and...

 that the Cassi along with other tribes such as the Ancalites

Ancalites

The Ancalites were a Celtic tribe, probably living around the Thames Valley area in the 1st century BC. They are known only from the writings of Julius Caesar....

 and Bibroci

Bibroci

The Bibroci were a tribe of Iron Age Britain encountered by Julius Caesar during his second expedition to Britain in 55 BC. They surrendered to him as he was campaigning against Cassivellaunus in the Thames Valley, which suggests they were also based in the south-east....

 merged to form the Catuvellauni

Catuvellauni

The Catuvellauni were a tribe or state of south-eastern Britain before the Roman conquest.The fortunes of the Catuvellauni and their kings before the conquest can be traced through numismatic evidence and scattered references in classical histories. They are mentioned by Dio Cassius, who implies...

, and that Cassivellaunus

Cassivellaunus

Cassivellaunus was an historical British chieftain who led the defence against Julius Caesar's second expedition to Britain in 54 BC. The first British person whose name is recorded, Cassivellaunus led an alliance of tribes against Roman forces, but eventually surrendered after his location was...

may have been a member of the Cassi tribe.