Julius Caesar made two campaigns to Britain, the first in 55 BC, and the next the following year. While not resulting in the conquest of any territory, they still brought at least part of the island within the influence of Rome.
Caligula planned his own campaign against the British in AD 40, but its execution was bizarre: according to Suetonius, he drew up his troops in battle formation facing the English Channel and ordered them to attack the standing water. Afterwards, he had the troops gather sea shells, referring to them as "plunder from the ocean, due to the Capitol and the Palace."
The actual Roman invasion of Britain had to wait until the reign of Claudius, in the year 44. The Roman troops defeated the British under Caratacus, and captured his capital Camulodunum or Colchester. Caratacus refused to submit, and retreated deeper into unconquered Britain, coming to the domain of the Ordovices[?] in 47. He incited this tribe to fight the Romans, and they lost the ensuing battle. Once again Caratacus fled, this time to Cartimandua, Queen of the Brigantes[?]. Cartimandua prudently surrendered Caratacus to the Romans, who brought him in chains to Rome. Thus ended the first phase of the conquest.
For the first twenty years, the Roman rule was oppressive, and this treatment forced Boudicca, Queen of the Iceni[?], to revolt. The Trinovantes[?] and Catuvellauni[?] joined, and the alliance assaulted the Roman colony at Camulodunum, looting and burning the town as well as slaying every man, woman and child they found. The governor Suetonius Paullinus, upon reaching London from his campaigning in the western part of the province, found the town indefensible with the few troops he had. As a result, Paullinus was forced to abandon the city and took only those who could afford to leave in time to retreat with him, leaving some behind. The Fourteenth legion joined him at a battlefield of his choosing, and the combined Roman forces crushed the revolt. Boudicca took her life shortly afterwards.
For much of the history of Roman Britain, there was a large number of soldiers garrisoned on the island. This required that the emperor station a trusted senior man as governor of the this province. As a side-effect of this, a number of future emperors served as governors of this province, including Vespasian, Pertinax, and Gordian I.
In the following years the Romans conquered more of the island, increasing the size of Roman Britain. The governor Agricola, father-in-law to the historian Tacitus, conquered the Ordovices[?] in 78. With the 20th legion, Agricola defeated the Caledonians[?] in AD 84 at the Battle of Mons Graupius[?], in what is today northern Scotland. This marked the high tide mark of Roman territory in Britain; shortly after his victory, Agricola was recalled from Britain back to Rome, and the Romans retired to a more defensible line along the Forth-Clyde isthmus, freeing soldiers badly needed along other frontiers of the Empire.
Twice in the second century, there were military crises in the province. The first incident was towards the end of Trajan's reign (117), which was handled by Q. Pompeius Falco. The second crisis was in 155 - 7, when the Brigantes revolted. This rebellion was suppressed by governor Julius Verus[?].
When Hadrian reached Britain on his famous tour of the Roman provinces around 120, he directed an extensive defensive wall, known as Hadrian's Wall to be built from the Tyne to the Solway Firth, which became the northern frontier of the province. This frontier was briefly extended forward to the Forth-Clyde isthmus in the reign of Antoninus Pius, where the Antonine Wall was built, from around 142 to the reign of Severus.
The usurpation of Albinus demonstrated the two major political problems posed by Roman Britain. First, in order to maintain its security, it had three legions stationed there, which would provide an ambitious man with weak loyalties a powerful base for rebellion -- which Albinus dutifully abused. Second, any rebellious official who used this resource must needs strip the island of its garrison to march on Rome and seize the throne, leaving the island defenseless to attackers -- which is what Albinus did in 196.
Following Albinus' defeat, Septimius Severus tried to solve this problem by dividing the existing province into two: Upper and Lower Britain. While this kept the potential for rebellion in check for almost a century, the revolt of Carausius (286-97) forced Constantius Chlorus, upon its suppression, to further divide the island into four provinces:
Constantius remained in Britain for the rest of the time he was part of the Tetrarchy, dying in Eburacum, present-day York, in 306. Constantine had managed to be by his side at that moment, and assumed his duties in Britain. Unlike the earlier usurper Albinus, he was able to successfully use his base in Britain as a starting point on his march to the imperial throne.
For a few years, the British provinces were loyal to the usurper Magnentius, who succeeded Constans following his death. Following his defeat and death in the Battle of Mon Seleucus[?] in 353, Constantius II dispatched his chief imperial notarary Paul Catena[?] to hunt down Magnentius' supporters. Paul's investigations deteriorated into a witch hunt, which forced the vicarus Flavius Martinus to intervene. When Paul instead suspected Martinus of treason, the vicarus found himself forced to physically attack Paul with a sword with the aim of assassinating him, but at the end committed suicide.
In the fourth century, Britain was also subjected to increasing outside attacks, the Saxons from the east, and the Irish from the west. A series of forts were built, starting around 280, to defend the coasts, but these preparations were not enough when a general assault of Saxons, Irish, and Attacotti[?] combined with a general revolt of the garrison on Hadrian's Wall, left Roman Britain supine in 367. This crisis was settled by Count Theodosius, father of future emperor Theodosius I.
Another usurper, Magnus Maximus, attempted to repeat Constantine's success by raising the standard of revolt in Seguntium[?] in 383, and bringing the troops across the Channel with him. His rebellion was ended in 388, but this time not all of the troops were returned to Britain by an empire that had suffered a great loss of life in the Battle of Adrianople in 378, and now was scrambling to find sufficient manpower to defend all of its borders.
The archaeological records of the final decades of Roman rule show undeniable signs of decay. Urban and villa life had grown less intense by the fourth quarter of the fourth century, pottery shards are not present in levels dating past AD 400, and coins minted past 402 are rare. So when Constantine III became Emperor in 407, and crossed the channel with the remaining units of the British garrison, effectively Roman Britain ended. The inhabitants were forced to look to their own defenses and government -- a fact made clear in a rescript the emperor Flavius Augustus Honorius sent them in 410.
Britain is also noteworthy as having the largest European region of the former Roman Empire which currently speaks neither:
For what is known of the process that introduced English to much of this former province, see the article Anglo-Saxons.