At the time of Julius Caesar's first small invasion of the south coast of Britain in 55 BC, the British Isles, like much of mainland Europe was inhabited by many Celtic tribes loosely united by a similar language and culture but nevertheless each distinct. He returned the next year and encountered the 4000 war chariots of the Catevellauni in a land "protected by forests and marshes, and filled with a great number of men and cattle." He defeated the Catevellauni and then withdrew, though not before establishing treaties and alliances. Thus began the Roman occupation of Britain.
Nearly 100 years later, in 43 AD, the Emperor Claudius sent Aulus Plautius and about 24,000 soldiers to Britain, this time to establish control under a military presence. Although subjugation of southern Britain proceeded fairly smoothly by a combination of military might and clever diplomacy, and by 79 AD what is now England and Wales were firmly under control, the far North remained a problem. However, the Emperor Vespasian decided that what is now Scotland should also be incorporated into the Roman Empire. Under his instructions the governor of Britian, Julius Agricola, subdued the Southern Scottish tribal clans, the Selgovae, Novantae and Votadini by 81 AD. Further to the North lived loose associations of clans known collectively as the Caledonians. Agricola tried to provoke them into battle by marching an army into the Highlands eventually forcing a battle with the Caledonian leader Calgacus in present day Aberdeenshire at a place called Mons Graupius. 30,000 Caledonians were killed, but the Roman victory was a hollow one, for the next day the surviving clansmen melted away into the hills, and were to remain fiercely resistant and independent.
By the time Hadrian became Emperor in 117 AD the Roman Empire had ceased to expand. Hadrian was concerned to consolidate his boundaries. He visited Britain in 122 AD, and ordered a wall to be built between the Solway Firth in the West and the River Tyne in the east "to separate Romans from Barbarians". One of the greatest monuments to the power- and limitations - of the Roman Empire, Hadrian's Wall ran for 73 miles across open country.