Julius Caesar invaded Britain in 55 and 54 BC as part of his Gallic Wars. The Britons had been overrun or culturally assimilated by other Celtic tribes during the British Iron Age and had been aiding Caesar’s enemies. He received tribute, installed a friendly king over the Trinovantes, and returned to Gaul. Planned invasions under Augustus were called off in 34, 27, and 25 BC. In AD 40, Caligula assembled 200,000 men at the Channel, only to have them gather seashells. Three years later, Claudius directed four legions to invade Britain and restore an exiled king over the Atrebates. The Romans defeated the Catuvellauni, and then organized their conquests as the Province of Britain (Latin: Provincia Britannia).
By the year 47, the Romans held the lands southeast of the Fosse Way. Control over Wales was delayed by reverses and the effects of Boudica‘s rebellion, but the Romans expanded steadily northward. 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 directly controlled. Around 197, the Severan Reforms divided Britain into two provinces: Upper and Lower (Britannia Superior and Inferior). During the Diocletian Reforms, at the end of the 3rd century, Britannia was divided into four provinces under the direction of a vicar, who administered the Diocese of the Britains. A fifth province, Valentia, is attested in the later 4th century.
For much of the later period of the Roman occupation, Britannia was subject to barbarian invasions and often came under the control of imperial usurpers and Imperial pretenders. The final Roman withdrawal from Britain occurred around 410; the native kingdoms are considered to have formed Sub-Roman Britain after that. Roman Britain (Latin: Britannia or, later, Britanniae, “the Britains”) was the areas of the island of Great Britain that were governed by the Roman Empire, from ad 43 to 409 or 410.
Following the conquest of the Britons, a distinctive Romano-British culture emerged as the Romans introduced improved agriculture, urban planning, industrial production, and architecture. After the initial invasions, Roman historians generally only mention Britain in passing. Thus, most present knowledge derives from archaeological investigations and occasional epigraphic evidence lauding the Britannic achievements of an emperor.
For more, see: Roman Britain