In the course of his Gallic Wars, Julius Caesar invaded Britain twice: in 55 and 54 BC. On the first occasion Caesar took with him only two legions, and achieved little beyond a landing on the coast of Kent. The second invasion consisted of 628 ships, five legions and 2,000 cavalry. The force was so imposing that the Britons did not dare contest Caesar's landing in Kent, waiting instead until he began to move inland. Caesar eventually penetrated into Middlesex and crossed the Thames, forcing the British warlord Cassivellaunus to surrender as a tributary to Rome and setting up Mandubracius of the Trinovantes as client king. Caesar included accounts of both invasions in his Commentarii de Bello Gallico, with the first significant first-hand descriptions of the people, culture and geography of the island. This is effectively the start of the written history, or at least the protohistory, of Britain.

The Roman conquest of Britain refers to the conquest of the island of Britain by occupying Roman forces. It began in earnest in AD 43 under Emperor Claudius, and was largely completed in the southern half of Britain by 87 when the Stanegate was established. Conquest of the far north and Scotland took longer with fluctuating success.

The Roman army was generally recruited in Italia, Hispania, and Gaul. To control the English Channel they used the newly formed fleet. The Romans under their general Aulus Plautius first forced their way inland in several battles against British tribes, including the Battle of the Medway, the Battle of the Thames, and in later years Caratacus's last battle and the Roman conquest of Anglesey. Following a widespread uprising in AD 60 in which Boudica sacked Camulodunum, Verulamium and Londinium, the Romans suppressed the rebellion in the Defeat of Boudica. They went on eventually to push as far north as central Caledonia in the Battle of Mons Graupius. Even after Hadrian's Wall was established as the border, tribes in Scotland and northern England repeatedly rebelled against Roman rule and forts continued to be maintained across northern Britain to protect against these attacks.

After capturing the south of the island, the Romans turned their attention to what is now Wales. The Silures, Ordovices and Deceangli remained implacably opposed to the invaders and for the first few decades were the focus of Roman military attention, despite occasional minor revolts among Roman allies like the Brigantes and the Iceni. The Silures were led by Caratacus, and he carried out an effective guerrilla campaign against Governor Publius Ostorius Scapula. Finally, in 51, Ostorius lured Caratacus into a set-piece battle and defeated him. The British leader sought refuge among the Brigantes, but their queen, Cartimandua, proved her loyalty by surrendering him to the Romans. He was brought as a captive to Rome, where a dignified speech he made during Claudius's triumph persuaded the emperor to spare his life. The Silures were still not pacified, and Cartimandua's ex-husband Venutius replaced Caratacus as the most prominent leader of British resistance.

The Romans invaded north-west Wales in 60/61 CE after subjugating much of southern Britain. Anglesey, recorded in Latin as Mona and still the island of Môn in modern Welsh, at the north-west corner of Wales, was a centre of resistance to Rome.

In 60/61 CE Suetonius Paulinus, Gaius Suetonius Paulinus, the conqueror of Mauretania (modern day Algeria and Morocco), became governor of Britannia. He led a successful assault to settle accounts with Druidism once and for all. Paulinus led his army across the Menai Strait and massacred the Druids and burnt their sacred groves.; he was drawn away by a revolt led by Boudica. The next invasion in 77 CE was led by Gnaeus Julius Agricola. It led to long-term Roman occupation. Both of these invasions of Anglesey were recorded by the Roman historian Tacitus.

The Boudican revolt was an armed uprising by native Celtic tribes against the Roman Empire. It took place c. 60-61 AD in the Roman province of Britain, and was led by Boudica, the Queen of the Iceni. The uprising was motivated by the Romans' failure to honour an agreement they had made with her husband, Prasutagus, regarding the succession of his kingdom upon his death, and by brutal mistreatment of Boudica and her daughters by the Romans. The revolt ended unsuccessfully after a decisive Roman victory at the Defeat of Boudica.

The earliest written record of a formal connection between Rome and Scotland is the attendance of the "King of Orkney" who was one of 11 British kings who submitted to the emperor Claudius at Colchester in AD 43 following the invasion of southern Britain three months earlier. The apparently cordial beginnings recorded in Colchester did not last. We know nothing of the foreign policies of the senior leaders in mainland Scotland in the 1st century, but by AD 71 the Roman governor Quintus Petillius Cerialis had launched an invasion.

The Votadini, who occupied the south-east of Scotland, came under Roman sway at an early stage and Cerialis sent one division north through their territory to the shores of the Firth of Forth. The Legio XX Valeria Victrix took a western route through Annandale in an attempt to encircle and isolate the Selgovae who occupied the central Southern Uplands. Early success tempted Cerialis further north and he began constructing a line of Glenblocker forts to the north and west of the Gask Ridge which marked a frontier between the Venicones to the south and the Caledonians to the north.

In the summer of AD 78 Gnaeus Julius Agricola arrived in Britain to take up his appointment as the new governor. Two years later his legions constructed a substantial fort at Trimontium near Melrose. Agricola is said to have pushed his armies to the estuary of the "River Taus" (usually assumed to be the River Tay) and established forts there, including a legionary fortress at Inchtuthil.

The total size of the Roman garrison in Scotland during the Flavian period of occupation is thought to have been some 25,000 troops, requiring 16–19,000 tons of grain per annum.

The Battle of Mons Graupius was, according to Tacitus, a Roman military victory in what is now Scotland, taking place in AD 83 or, less probably, 84. The exact location of the battle is a matter of debate. Historians have long questioned some details of Tacitus's account of the fight, suggesting that he exaggerated Roman success.

This was the high-water mark of Roman territory in Britain. Following this final battle, it was proclaimed that Agricola had finally subdued all the tribes of Britain. Soon afterwards he was recalled to Rome, and his post passed to Sallustius Lucullus. It is likely that Rome intended to continue the conflict, but that military requirements elsewhere in the empire necessitated a troop withdrawal and the opportunity was lost.

Hadrian's Wall (Latin: Vallum Aelium), also known as the Roman Wall, Picts' Wall, or Vallum Hadriani in Latin, is a former defensive fortification of the Roman province of Britannia, begun in AD 122 in the reign of the emperor Hadrian. Running "from Wallsend on the River Tyne in the east to Bowness-on-Solway in the west", the Wall covered the whole width of the island. In addition to the wall's defensive military role, its gates may have been customs posts.

A significant portion of the wall still stands and can be followed on foot along the adjoining Hadrian's Wall Path. The largest Roman archaeological feature in Britain, it runs a total of 73 miles (117.5 kilometres) in northern England. Regarded as a British cultural icon, Hadrian's Wall is one of Britain's major ancient tourist attractions. It was designated as a UNESCO World Heritage Site in 1987. In comparison, the Antonine Wall, thought by some to be based on Hadrian's wall, was not declared a World Heritage site until 2008.Hadrian's Wall marked the boundary between Roman Britannia and unconquered Caledonia to the north. The wall lies entirely within England and has never formed the Anglo-Scottish border.

Quintus Lollius Urbicus was made governor of Roman Britain in 138, by the new emperor Antoninus Pius. Antoninus Pius soon reversed the containment policy of his predecessor Hadrian, and Urbicus was ordered to begin the reconquest of Lowland Scotland by moving north. Between 139 and 140 he rebuilt the fort at Corbridge and by 142 or 143, commemorative coins were issued celebrating a victory in Britain. It is therefore likely that Urbicus led the reoccupation of southern Scotland c. 141, probably using the 2nd Augustan Legion. He evidently campaigned against several British tribes (possibly including factions of the northern Brigantes), certainly against the lowland tribes of Scotland, the Votadini and Selgovae of the Scottish Borders region, and the Damnonii of Strathclyde. His total force may have been about 16,500 men.

It seems likely that Urbicus planned his campaign of attack from Corbridge, advancing north and leaving garrison forts at High Rochester in Northumberland and possibly also at Trimontium as he struck towards the Firth of Forth. Having secured an overland supply route for military personnel and equipment along Dere Street, Urbicus very likely set up a supply port at Carriden for the supply of grain and other foodstuffs before proceeding against the Damnonii; success was swift.

The Antonine Wall, known to the Romans as Vallum Antonini, was a turf fortification on stone foundations, built by the Romans across what is now the Central Belt of Scotland, between the Firth of Forth and the Firth of Clyde. Built some twenty years after Hadrian's Wall to the south, and intended to supersede it, while it was garrisoned it was the northernmost frontier barrier of the Roman Empire. It spanned approximately 63 kilometres (39 miles) and was about 3 metres (10 feet) high and 5 metres (16 feet) wide. Lidar scans have been carried out to establish the length of the wall and the Roman distance units used. Security was bolstered by a deep ditch on the northern side. It is thought that there was a wooden palisade on top of the turf. The barrier was the second of two "great walls" created by the Romans in Great Britain in the second century AD. Its ruins are less evident than those of the better-known and longer Hadrian's Wall to the south, primarily because the turf and wood wall has largely weathered away, unlike its stone-built southern predecessor.

The Antonine Wall had a variety of purposes. It provided a defensive line against the Caledonians. It cut off the Maeatae from their Caledonian allies and created a buffer zone north of Hadrian's Wall. It also facilitated troop movements between east and west, but its main purpose may not have been primarily military. It enabled Rome to control and tax trade and may have prevented potentially disloyal new subjects of Roman rule from communicating with their independent brethren to the north and coordinating revolts. Urbicus achieved an impressive series of military successes, but like Agricola's they were short-lived. Construction began in 142 AD at the order of Roman Emperor Antoninus Pius, and took about 12 years to complete. Having taken twelve years to build, the wall was overrun and abandoned soon after AD 160. The wall was abandoned only eight years after completion, and the garrisons relocated rearward to Hadrian's Wall.

Pressure from the Caledonians may have led Antoninus to send the empire's troops farther north. The Antonine Wall was protected by 16 forts with small fortlets between them; troop movement was facilitated by a road linking all the sites known as the Military Way. The soldiers who built the wall commemorated the construction and their struggles with the Caledonians with decorative slabs, twenty of which survive.

In 175, a large force of Sarmatian cavalry, consisting of 5,500 men, arrived in Britannia, probably to reinforce troops fighting unrecorded uprisings. In 180, Hadrian's Wall was breached by the Picts and the commanding officer or governor was killed there in what Cassius Dio described as the most serious war of the reign of Commodus. Ulpius Marcellus was sent as replacement governor and by 184 he had won a new peace, only to be faced with a mutiny from his own troops. Unhappy with Marcellus's strictness, they tried to elect a legate named Priscus as usurper governor; he refused, but Marcellus was lucky to leave the province alive. The Roman army in Britannia continued its insubordination: they sent a delegation of 1,500 to Rome to demand the execution of Tigidius Perennis, a Praetorian prefect who they felt had earlier wronged them by posting lowly equites to legate ranks in Britannia. Commodus met the party outside Rome and agreed to have Perennis killed, but this only made them feel more secure in their mutiny.

The future emperor Pertinax was sent to Britannia to quell the mutiny and was initially successful in regaining control, but a riot broke out among the troops. Pertinax was attacked and left for dead, and asked to be recalled to Rome, where he briefly succeeded Commodus as emperor in 192.

The Roman frontier became Hadrian's Wall again, although Roman incursions into Scotland continued. Initially, outpost forts were occupied in the south-west and Trimontium remained in use but they too were abandoned after the mid-180s. Roman troops, however, penetrated far into the north of modern Scotland several more times. Indeed, there is a greater density of Roman marching camps in Scotland than anywhere else in Europe, as a result of at least four major attempts to subdue the area. The Antonine Wall was occupied again for a brief period after AD 197. The most notable invasion was in 209 when the emperor Septimius Severus, claiming to be provoked by the belligerence of the Maeatae, campaigned against the Caledonian Confederacy. Severus invaded Caledonia with an army perhaps over 40,000 strong. According to Dio Cassius, he inflicted genocidal depredations on the natives and incurred the loss of 50,000 of his own men to the attrition of guerrilla tactics, although it is likely that these figures are a significant exaggeration.

By 210, Severus' campaigning had made significant gains, but his campaign was cut short when he fell fatally ill, dying at Eboracum in 211. Although his son Caracalla continued campaigning the following year, he soon settled for peace. The Romans never campaigned deep into Caledonia again: they soon withdrew south permanently to Hadrian's Wall. From the time of Caracalla onwards, no further attempts were made to permanently occupy territory in Scotland.

The death of Commodus put into motion a series of events which eventually led to civil war. Following the short reign of Pertinax, several rivals for the emperorship emerged, including Septimius Severus and Clodius Albinus. The latter was the new governor of Britannia, and had seemingly won the natives over after their earlier rebellions; he also controlled three legions, making him a potentially significant claimant. His sometime rival Severus promised him the title of Caesar in return for Albinus's support against Pescennius Niger in the east. Once Niger was neutralised, Severus turned on his ally in Britannia — it is likely that Albinus saw he would be the next target and was already preparing for war.

Albinus crossed to Gaul in 195, where the provinces were also sympathetic to him, and set up at Lugdunum. Severus arrived in February 196, and the ensuing battle was decisive. Albinus came close to victory, but Severus's reinforcements won the day, and the British governor committed suicide. Severus soon purged Albinus's sympathisers and perhaps confiscated large tracts of land in Britain as punishment.

Albinus had demonstrated the major problem posed by Roman Britain. In order to maintain security, the province required the presence of three legions; but command of these forces provided an ideal power base for ambitious rivals. Deploying those legions elsewhere would strip the island of its garrison, leaving the province defenceless against uprisings by the native Celtic tribes and against invasion by the Picts and Scots.

The Roman invasion of Caledonia was launched in 208 by the Roman emperor Septimius Severus. The invasion lasted until late 210 when the emperor became ill and died at Eboracum (York) on 4 February 211. The war started well for the Romans with Severus managing to quickly reach the Antonine Wall, but when Severus pushed north into the highlands he became bogged down in a guerrilla war and he was never able to fully subjugate Caledonia. He reoccupied many forts built by Agricola over 100 years earlier, following the Battle of Mons Graupius, and crippled the ability of the Caledonians to raid Roman Britain. The invasion was abandoned by Severus' son Caracalla and Roman forces once again withdrew to Hadrian's Wall.

Although Caracalla withdrew from all the territory taken during the war, the latter did have some practical benefits for the Romans. These include the rebuilding of Hadrian's Wall which once again became the border of Roman Britain. The war also led to the reinforcing of the British frontier, which had been in dire need of reinforcements, and to the weakening of the various Caledonian tribes. It would take many years for them to recover their strength and begin raiding in strength.

The Carausian revolt (AD 286–296) was an episode in Roman history, during which a Roman naval commander, Carausius, declared himself emperor over Britain and northern Gaul. His Gallic territories were retaken by the western Caesar Constantius Chlorus in 293, after which Carausius was assassinated by his subordinate Allectus. Britain was regained by Constantius and his subordinate Asclepiodotus in 296.

Britannia Prima or Britannia I (Latin for "First Britain") was one of the provinces of the Diocese of "the Britains" created during the Diocletian Reforms at the end of the 3rd century. It was probably created after the defeat of the usurper Allectus by Constantius Chlorus in AD 296 and was mentioned in the c. 312 Verona List of the Roman provinces. Its position and capital remain uncertain, although it was probably located closer to Rome than Britannia II. At present, most scholars place Britannia I in Wales, Cornwall, and the lands connecting them. On the basis of a recovered inscription, its capital is now usually placed at Corinium of the Dobunni (Cirencester) but some emendations of the list of bishops attending the 315 Council of Arles would place a provincial capital in Isca (Caerleon) or Deva (Chester), which were known legionary bases.

Emperor Constantius returned to Britain in 306, despite his poor health, with an army aiming to invade northern Britain, the provincial defences having been rebuilt in the preceding years. Little is known of his campaigns with scant archaeological evidence, but fragmentary historical sources suggest he reached the far north of Britain and won a major battle in early summer before returning south. His son Constantine (later Constantine the Great) spent a year in northern Britain at his father's side, campaigning against the Picts beyond Hadrian's Wall in the summer and autumn. Constantius died in York in July 306 with his son at his side. Constantine then successfully used Britain as the starting point of his march to the imperial throne, unlike the earlier usurper, Albinus.

Britannia Secunda or Britannia II (Latin for "Second Britain") was one of the provinces of the Diocese of "the Britains" created during the Diocletian Reforms at the end of the 3rd century. It was probably created after the defeat of the usurper Allectus by Constantius Chlorus in AD 296 and was mentioned in the c. 312 Verona List of the Roman provinces. Its position and capital remain uncertain, although it probably lay further from Rome than Britannia I. At present, most scholars place Britannia II in Yorkshire and northern England. If so, its capital would have been Eboracum (York).

In the winter of 367, the Roman garrison on Hadrian's Wall apparently rebelled, and allowed Picts from Caledonia to enter Britannia. Simultaneously, Attacotti, the Scotti from Hibernia, and Saxons from Germania landed in what might have been coordinated and pre-arranged waves on the island's mid-western and southeastern borders, respectively. Franks and Saxons also landed in northern Gaul.

These warbands managed to overwhelm nearly all of the loyal Roman outposts and settlements. The entire western and northern areas of Britannia were overwhelmed, the cities sacked and the civilian Romano-British murdered, raped, or enslaved.

Nectaridus, the comes maritime tractus (commanding general of the sea coast region), was killed and the Dux Britanniarum, Fullofaudes, was either besieged or captured and the remaining loyal army units stayed garrisoned inside southeastern cities.

The miles areani or local Roman agents that provided intelligence on barbarian movements seem to have betrayed their paymasters for bribes, making the attacks completely unexpected. Deserting soldiers and escaped slaves roamed the countryside and turned to robbery to support themselves. Although the chaos was widespread and initially concerted, the aims of the rebels were simply personal enrichment and they worked as small bands rather than larger armies.

Another imperial usurper, Magnus Maximus, raised the standard of revolt at Segontium (Caernarfon) in north Wales in 383, and crossed the English Channel. Maximus held much of the western empire, and fought a successful campaign against the Picts and Scots around 384. His continental exploits required troops from Britain, and it appears that forts at Chester and elsewhere were abandoned in this period, triggering raids and settlement in north Wales by the Irish. His rule was ended in 388, but not all the British troops may have returned: the Empire's military resources were stretched to the limit along the Rhine and Danube. Around 396 there were more barbarian incursions into Britain. Stilicho led a punitive expedition. It seems peace was restored by 399, and it is likely that no further garrisoning was ordered; by 401 more troops were withdrawn, to assist in the war against Alaric I.

By the early 5th century, the Roman Empire could no longer defend itself against either internal rebellion or the external threat posed by Germanic tribes expanding in Western Europe. This situation and its consequences governed the eventual permanent detachment of Britain from the rest of the Empire. After a period of local self-rule the Anglo-Saxons came to southern England in the 440s.

The end of Roman rule in Britain was the transition from Roman Britain to post-Roman Britain. Roman rule ended in different parts of Britain at different times, and under different circumstances.

In 383, the usurper Magnus Maximus withdrew troops from northern and western Britain, probably leaving local warlords in charge. Around 410, the Romano-British expelled the magistrates of the usurper Constantine III. He had previously stripped the Roman garrison from Britain and taken it to Gaul in response to the Crossing of the Rhine in late 406, leaving the island a victim to barbarian attacks. Roman Emperor Honorius replied to a request for assistance with the Rescript of Honorius, telling the Roman cities to see to their own defence, a tacit acceptance of temporary British self-government. Honorius was fighting a large-scale war in Italy against the Visigoths under their leader Alaric, with Rome itself under siege. No forces could be spared to protect distant Britain. Though it is likely that Honorius expected to regain control over the provinces soon, by the mid-6th century Procopius recognised that Roman control of Britannia was entirely lost.

During their occupation of Britain the Romans built an extensive network of roads which continued to be used in later centuries and many are still followed today. The Romans also built water supply, sanitation and wastewater systems. Many of Britain's major cities, such as London (Londinium), Manchester (Mamucium) and York (Eboracum), were founded by the Romans, but the original Roman settlements were abandoned not long after the Romans left.

Unlike many other areas of the Western Roman Empire, the current majority language is not a Romance language, or a language descended from the pre-Roman inhabitants. The British language at the time of the invasion was Common Brittonic, and remained so after the Romans withdrew. It later split into regional languages, notably Cumbric, Cornish, Breton and Welsh. Examination of these languages suggests some 800 Latin words were incorporated into Common Brittonic (see Brittonic languages). The current majority language, English, is based on the languages of the Germanic tribes who migrated to the island from continental Europe from the 5th century onwards.