Mandubracius in the context of Britannia (Roman province)

Roman Britain was the territory that became the Roman province of Britannia after the Roman conquest of Britain, consisting of a large part of the island of Great Britain. The occupation lasted from AD 43 to AD 410.

Britain was invaded by Julius Caesar in 55 and 54 BC as part of the Gallic Wars. According to Caesar, the Britons had been overrun or culturally assimilated by the Belgae during the British Iron Age and had been aiding Caesar's enemies. The Belgae were the only Celtic tribe to cross the sea into Britain, for to all other Celtic tribes this land was unknown. He received tribute, installed the Celtic king Mandubracius over the Trinovantes, and returned to Gaul. Planned invasions under Augustus were called off in 34, 27, and 25 BC. In 40 AD, Caligula assembled 200,000 men at the Channel on the continent, only to have them gather seashells (musculi) according to Suetonius, perhaps as a symbolic gesture to proclaim Caligula's victory over the sea. Three years later, Claudius directed four legions to invade Britain and restore the exiled king Verica over the Atrebates. The Romans defeated the Catuvellauni, and then organized their conquests as the province of Britain. By 47 AD, the Romans held the lands southeast of the Fosse Way. Control over Wales was delayed by reverses and the effects of Boudica's uprising, but the Romans expanded steadily northwards.

In the course of his Gallic Wars, Julius Caesar invaded Britain twice: in 55 and 54 BC. On the first occasion, Caesar took with him only two legions, and achieved little beyond a landing on the coast of Kent. The second invasion was more substantial, consisting of 800 ships, five legions and 2,000 cavalry. The force was so imposing that the Celtic Britons did not contest Caesar's landing, waiting instead until he began to move inland. Caesar eventually penetrated into Middlesex and crossed the Thames, forcing the British warlord Cassivellaunus to pay tribute to Rome and setting up Mandubracius of the Trinovantes as a client king. The Romans then returned to Gaul without conquering any territory.

Caesar included accounts of both invasions in his Commentarii de Bello Gallico, which contains the earliest surviving significant eyewitness descriptions of the island's people, culture and geography. This is effectively the start of the written history, or at least the protohistory, of Great 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.