Julius Caesar invaded Britain in 55 and 54 BC as part of his Gallic Wars. According to Caesar, 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 40 AD, 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 uprising, 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 controlled. Around 197, the Severan Reforms divided Britain into two provinces: Britannia Superior and Britannia Inferior. 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. 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.
Following the conquest of the Britons, a distinctive Romano-British culture emerged as the Romans introduced improved agriculture, urban planning, industrial production, and architecture. The Roman goddess Britannia became the female personification of Britain. 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.:46,323 Roman citizens settled in Britain from many parts of the Empire.
Britain was known to the Classical world; the Greeks, Phoenicians and Carthaginians traded for Cornish tin in the 4th century BC. The Greeks referred to the Cassiterides, or "tin islands", and placed them near the west coast of Europe. The Carthaginian sailor Himilco is said to have visited the island in the 5th century BC and the Greek explorer Pytheas in the 4th. However, it was regarded as a place of mystery, with some writers refusing to believe it existed at all.
The first direct Roman contact was when Julius Caesar undertook two expeditions in 55 and 54 BC, as part of his conquest of Gaul, believing the Britons were helping the Gallic resistance. The first expedition was more a reconnaissance than a full invasion and gained a foothold on the coast of Kent but was unable to advance further because of storm damage to the ships and a lack of cavalry. Despite the military failure it was a political success, with the Roman Senate declaring a 20-day public holiday in Rome to honour the unprecedented achievement of obtaining hostages from Britain and defeating Belgian tribes on returning to the continent.