Lesson I · 55 BC to AD 47

The invasion

The questionWhy did Rome invade Britain in AD 43, and how did it win?

Julius Caesar came twice and left. A century later, a new emperor who badly needed a victory sent an army of about 40,000 men across the Channel. This time they stayed.

Emperor
Claudius, AD 41 to 541
Army
about 40,000 men
Commander
Aulus Plautius
Claudius in Britain
16 days

Fits: Primary (KS2) · GCSE · A level

Grassy parallel ditches curving across a field inside ruined walls
Ditches at Richborough in Kent. The earliest, two parallel V-shaped ditches, protected the invasion beachhead of AD 43.Photo: Colin Smith, CC BY-SA 2.0, via Geograph

The story

Julius Caesar, who was conquering Gaul, crossed to Britain twice, in 55 and 54 BC. He fought, took tribute from some tribes and learned about the south and its riches, but he did not come to conquer, and he did not stay.1

In the century after, Rome and Britain traded. Some British peoples, like the Atrebates in the south, became friendly with Rome; others, above all the Catuvellauni north of the Thames, did not.1

Why Claudius came

In AD 41 the emperor Caligula was murdered, and the Praetorian Guard made his uncle Claudius emperor. Claudius had no military record and needed to secure his throne. Caligula had gathered an army to invade Britain, then called it off, which the army took as a humiliation. When the Catuvellauni drove out Verica, king of the friendly Atrebates, Claudius may have used it as his excuse.1

The real reasons were probably the ones emperors usually had: glory, popularity, loot, Britain’s resources, and an army kept busy and pleased.1

The crossing

In AD 43 the army, under the general Aulus Plautius, sailed from Gaul, probably from Boulogne, in three divisions, says Dio. It was about 40,000 strong, and moving it may have needed 700 to 1,000 ships. Where it landed is still argued over; Richborough in Kent is the most widely accepted answer, with Chichester the main rival.1 At Richborough, the army dug two long parallel ditches to protect its beachhead. They are still there.2

Two battles and an emperor

Dio describes two great battles. The first was at a river, probably the Medway, where auxiliary cavalry swam their horses across to attack the Britons from the side. The second, on the Thames, beat the Catuvellauni.1

Then Claudius himself arrived, with soldiers of the Praetorian Guard and, according to Dio, elephants, for the final march on Colchester, the Catuvellauni’s capital. Dio says eleven British kings surrendered to him. One leader who did not, Caratacus, escaped to fight on for years. Claudius stayed for just sixteen days. In Rome the Senate voted him a triumph, arches, and the title Britannicus.1

Four years on

By the end of AD 47, the army had spread across much of lowland Britain, into the Midlands and the south-west, building forts as it went. The future emperor Vespasian, then commanding a legion, campaigned in the south-west and took the Isle of Wight. Some peoples were won over without fighting: East Anglia, home of the Iceni, became a friendly kingdom.1 That arrangement would end in fire, thirteen years later.

Near a crossing of the Thames, the Romans had built a small fort. Within a few years a new town grew there: Londinium.3

The army of AD 43

About 40,000 men, counted in thousands.

  1. Legionaries

    Four legions of heavy infantry, all Roman citizens: about 20,000.

  2. Auxiliaries

    “A similar number” of non-citizen soldiers, including cavalry: about 20,000 more.

One block is one thousand men. Dashed: possibly. English Heritage’s estimate. Only two of the four legions can be named for certain: the Second, Augusta, under Vespasian, and the Twentieth.

The first four years

Places named in the story, AD 43 to 47.

RichboroughChichester?The Medway?ColchesterAlchesterLongthorpe?Hod HillLake FarmIsle of WightGaul
Southern Britain with today’s coast. A question mark means the place is probable, not certain. The Thames battle is left off: its site is not known.4
  1. Richborough Where the army most probably landed in AD 43, and dug ditches to guard the beachhead.
  2. Chichester? The other likely landing place, in the lands of the Atrebates, a British people friendly to Rome.
  3. The Medway? The first great battle was at a river crossing, probably here. Cavalry swam their horses across.
  4. Colchester Capital of the Catuvellauni, taken with Claudius there in person.
  5. Alchester A base behind the army’s advance.
  6. Longthorpe? A fort near Peterborough that may be older than AD 47.
  7. Hod Hill A Roman fort built inside a hillfort of the Durotriges.
  8. Lake Farm A legionary fortress near Wimborne, in the south-west campaign.
  9. Isle of Wight Taken by Vespasian, the future emperor, says Suetonius.

Key words

Legion
A unit of about 5,000 Roman citizen soldiers, trained as heavy infantry.
Auxiliaries
Soldiers who were not Roman citizens, often recruited from conquered peoples, including the cavalry.
Catuvellauni
The most powerful British people in the south-east in AD 43, with their capital at Colchester.
Pretext
An excuse given for an action, which hides the real reason. Claudius may have used the exile of a British king, Verica, as one.
Beachhead
Ground held on a shore so that more troops and supplies can land safely.

Check yourself

Try to answer in your head, then open each one.

Did Julius Caesar conquer Britain?

No. He came in 55 and 54 BC, took tribute from some tribes and learned about the south, but he did not intend to conquer it, and did not.

Give two reasons why Claudius wanted to invade.

Any two of: to make his position as emperor safe, glory and popularity in Rome, to please the army after Caligula called off his invasion, booty, and Britain’s resources.

Where did the army probably land, and what is the other main idea?

Richborough in Kent is the most widely accepted. Chichester, on the south coast, is the main alternative.

How long did Claudius himself stay in Britain?

Only 16 days, according to Cassius Dio.

Why are historians unsure about so much of the invasion?

No Roman account was written at the time. The longest, by Cassius Dio, was written about 150 years later, and the sources are few and brief.

Go deeper

Sources for this page

Every number on this page comes from one of these. How we check.

Last reviewed 12 November 2026. Spotted a mistake? Tell us.

  1. The Roman invasion of Britain, AD 43. English Heritage, Story of England: Romans. www.english-heritage.org.uk/learn/story-of-england/romans/invasion/Used for: Why Claudius invaded, the size of the army, and the debate over where it landed. Checked 12 November 2026.
  2. History of Richborough Roman Fort and Amphitheatre. English Heritage. www.english-heritage.org.uk/visit/places/richborough-roman-fort-and-amphitheatre/history-and-stories/history/Used for: Richborough: the landing of AD 43, the coastline, the port, the amphitheatre and the Saxon Shore fort. Checked 12 November 2026.
  3. London Museum. A timeline of Roman London: Londinium from start to end. www.londonmuseum.org.uk/collections/london-stories/timeline-roman-london-londinium/Used for: Londinium: founding, population, the fires, the wall, the provinces and the end. Checked 12 November 2026.
  4. Natural Earth, 1:110m and 1:50m land, public domain, via the world-atlas package by Mike Bostock (ISC licence). www.naturalearthdata.com/Used for: Every coastline drawn on this site. Checked 11 November 2026.