

A fifth province, Valentia, is attested in the later 4th century. During the Diocletian Reforms, at the end of the 3rd century, Britannia was divided into four provinces under the direction of a vicarius, who administered the Diocese of the Britains. Around 197 AD, the Severan Reforms divided Britain into two provinces: Britannia Superior and Britannia Inferior.

Under the 2nd-century emperors Hadrian and Antoninus Pius, two walls were built to defend the Roman province from the Caledonians, whose realms in the Scottish Highlands were never controlled. In the context of pre-industrial warfare and of a total population of Britain of c. The bloodbath at Mons Graupius concluded the forty-year conquest of Britain, a period that possibly saw between 100,000 and 250,000 Britons killed.

Battle casualties were estimated by Tacitus to be upwards of 10,000 on the Caledonian side and about 360 on the Roman side. In mid-84 AD, Agricola faced the armies of the Caledonians, led by Calgacus, at the Battle of Mons Graupius. The conquest of Britain continued under command of Gnaeus Julius Agricola (77–84), who expanded the Roman Empire as far as Caledonia. Control over Wales was delayed by reverses and the effects of Boudica's uprising, but the Romans expanded steadily northward. By 47 AD, the Romans held the lands southeast of the Fosse Way. The Romans defeated the Catuvellauni, and then organized their conquests as the Province of Britain. Three years later, Claudius directed four legions to invade Britain and restore the exiled king Verica over the Atrebates. In 40 AD, Caligula assembled 200,000 men at the Channel on the continent, only to have them gather seashells ( musculi) according to Suetonius, perhaps as a symbolic gesture to proclaim Caligula's victory over the sea.

Planned invasions under Augustus were called off in 34, 27, and 25 BC. He received tribute, installed the friendly king Mandubracius over the Trinovantes, and returned to Gaul. The Belgae were the only Celtic tribe to cross the sea into Britain, for to all other Celtic tribes this land was unknown. According to Caesar, the Britons had been overrun or culturally assimilated by Belgic tribes during the British Iron Age and had been aiding Caesar's enemies. Julius Caesar invaded Britain in 55 and 54 BC as part of his Gallic Wars. The occupation lasted from AD 43 to AD 410. Roman Britain was the territory that became the Roman Province of Britannia after the Roman conquest of Britain, consisting of a large part of the island of Great Britain.
