The Claudian force for the invasion in 42 CE consisted of four legions drawn from the Rhine and Danube frontiers. Auxiliary infantry and cavalry units accompanied the legions. The legionary force will have numbered around 25,000 men. With auxiliaries (mainly cavalry) the total could have reached 40,000.

Aulus Plautius was in command with orders to send for the Emperor when in sight of victory. Claudius was prepared to join his forces in Britain with his Praetorian Guard, Legio VIII and a detachment of elephants.

Having assembled at Boulogne, the force made the crossing in three divisions. A landing was made unopposed at Richborough, where there is evidence of a beach head, and possibly but not probably, at other points on the south coast. Evidence of Claudian occupation has been found in Reculver. The primary objective of the invasion being to destroy the power of the Catuvellauni, it was imperative to advance to the Medway and the Thames and capture their capital Camulodunum (near Colchester).

Cunobelin had expelled Verica from his Atrebatan territory thus giving a pretext for Roman intervention. A sept of the Dobunni of the Severn valley surrendered to Plautius when he reached the Medway.

Resistance was led by Caratacus and Togodumnus sons of Cunobelin. The battle for the Medway crossing lasted two days. Plautius used the Batavi to swim the river and create confusion while Vespasian crossed with Legio II Augusta some distance from the main force. Strong Roman attacks on the second day led by Hosidius Geta forced the Britons to fall back to the Thames. The Thames was crossed with the Batavi again leading the way.