In ad43 the Roman Empire invaded Briton. At first the invading army occupied only southern and eastern areas of the island, but soon the legions would march again to conquer nearly all of Great Britain.
In the decades following the raids by Julius Caesar, the British tribes had established firm links with the Roman Empire just across the Channel. Wheat, gold, slaves and furs were traded in exchange for pottery, wine and other luxuries. Some tribes, including the Atrebates of southern Britain and the Trinovantes of East Anglia, may have signed treaties of friendship with Rome.
Despite such links, the Celtic tribes retained their independence and continued their warfare and rivalry. By ad40 the most powerful was the Catuvellauni under their king Caractacus. In ad42 the Catuvellauni attacked the Atrebates, for reasons that are now obscure. After seeing his armies defeated, King Verica of the Atrebates fled to Gaul and sent an appeal to Rome for help against his enemy.
Verica’s message arrived at the right time. Emperor Claudius had just been voted the title and regalia of a successful general by the Senate, but he had not actually won a great victory. Claudius seized on the appeal of Verica as the excuse for a war against the Catuvellauni. Claudius sent his chief secretary, Narcissus, to northern Gaul to prepare the invasion force.
In the summer of 43 the invasion force landed near Rochester under the command o of the veteran general Aulus Plautius. The invasion force was made up of four legions, about 25,000 men, together with another 20,000 auxiliary forces. These auxiliaries were cavalry, light infantry and specialist engineers. The Catuvellauni and their allies could call upon many thousands of warriors to face this force, both cavalry and infantry. But the Celts lacked the military discipline of the Romans and were only part-time soldiers who also had farms to look after.
Aulus Plautius ran into a large force of Britons under Caractacus when he tried to cross the River Medway near Rochester. After savage fighting a force of auxiliary engineers, probably from Batavia, crossed the river and formed a bridgehead for the legions to occupy. The British withdrew north of the Thames.