The Romans invaded England in AD 43 with a large army of experienced troops. There were four legions, each of about 5000 men; the IInd Augusta, XIVth Gemina, XXth, and the IXth Hispana. They were accompanied by about 20,000 auxiliaries, probably from the Rhine army or the Gallic provinces; many of these were cavalry units recruited from places which had a tradition of fighting on horseback.

The Romans were well-armed and well-protected. Legionary soldiers, who fought in formations, carried a short, stabbing sword (gladius), a dagger for close fighting and a javelin ( pilum), which could be thrown or used in hand-to-hand fighting. They wore metal helmets and body armour and carried shields which could be held close together to form a protective wall and roof. Auxiliaries were sometimes more lightly armed because they had to move quickly, scouting ahead of the main body of troops, attacking on the flanks and pursuing fleeing enemies.

The British tribes were brave and experienced at chariot warfare, but many fought without armour and were very loosely organised. Tribal leaders could call on the loyalty of their nobles to fight for them, but there was no permanent army. Against the professional, battle-hardened Roman army supplied with superior weaponry, the British were completely overwhelmed.