By AD 43, the time of the main Roman invasion of Britain, Great Britain had already frequently been the target of invasions, planned and actual, by forces of the Roman Republic and Roman Empire. In common with other regions on the edge of the empire, Britain had enjoyed diplomatic and trading links with the Romans in the century since Julius Caesar's expeditions in 55 and 54 BC, and Roman economic and cultural influence was a significant part of the British late pre-Roman Iron Age, especially in the south.
Between 55 BC and the 40s AD, the status quo of tribute, hostages, and client states without direct military occupation, begun by Caesar's invasions of Britain, largely remained intact. Augustus prepared invasions in 34 BC, 27 BC and 25 BC. The first and third were called off due to revolts elsewhere in the empire, the second because the Britons seemed ready to come to terms. According to Augustus's Res Gestae, two British kings, Dumnovellaunus and Tincomarus, fled to Rome as suppliants during his reign, and Strabo's Geography, written during this period, says that Britain paid more in customs and duties than could be raised by taxation if the island were conquered.
By the 40s AD, however, the political situation within Britain was apparently in foment. The Catuvellauni had displaced the Trinovantes as the most powerful kingdom in south-eastern Britain, taking over the former Trinovantian capital of Camulodunum (Colchester), and were pressing their neighbours the Atrebates, ruled by the descendants of Julius Caesar's former ally Commius.
Caligula planned a campaign against the British in 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". Modern historians are unsure if that was meant to be an ironic punishment for the soldiers' mutiny or due to Caligula's derangement. Certainly this invasion attempt readied the troops and facilities that would make Claudius' invasion possible 3 years later (e.g. a lighthouse was built by Caligula at Boulogne-sur-Mer, the model for the one built soon after 43 at Dubris).
The II Augusta is known to have been commanded by the future emperor Vespasian. Three other men of appropriate rank to command legions are known from the sources to have been involved in the invasion. Gnaeus Hosidius Geta, who probably led the IX Hispana, and Vespasian's brother Titus Flavius Sabinus II are mentioned by Dio Cassius (Dio says that Sabinus was Vespasian's lieutenant, but as Sabinus was the older brother and preceded Vespasian into public life, he could hardly have been a military tribune). Gnaeus Sentius Saturninus is mentioned by Eutropius, although as a former consul he may have been too senior, and perhaps accompanied Claudius later.
The main invasion force under Aulus Plautius crossed in three divisions. The port of departure is usually taken to have been Boulogne, and the main landing at Rutupiae (Richborough, on the east coast of Kent). Neither of these locations is certain. Dio does not mention the port of departure, and although Suetonius says that the secondary force under Claudius sailed from Boulogne, it does not necessarily follow that the entire invasion force did. Richborough has a large natural harbour which would have been suitable, and archaeology shows Roman military occupation at about the right time. However, Dio says the Romans sailed east to west, and a journey from Boulogne to Richborough is south to north. Some historians suggest a sailing from Boulogne to the Solent, landing in the vicinity of Noviomagus (Chichester) or Southampton, in territory formerly ruled by Verica. An alternative explanation might be a sailing from the mouth of the Rhine to Richborough, which would be east to west.
The British were pushed back to the Thames. The Romans pursued them across the river causing them to lose men in the marshes of Essex. Whether the Romans made use of an existing bridge for this purpose or built a temporary one is uncertain. At least one division of auxiliary Batavian troops swam across the river as a separate force.
Togodumnus died shortly after the battle on the Thames. Plautius halted and sent word for Claudius to join him for the final push. Cassius Dio presents this as Plautius needing the emperor's assistance to defeat the resurgent British, who were determined to avenge Togodumnus. However, Claudius was no military man. Claudius's arch says he received the surrender of eleven kings without any loss, and Suetonius says that Claudius received the surrender of the Britons without battle or bloodshed. It is likely that the Catuvellauni were already as good as beaten, allowing the emperor to appear as conqueror on the final march on Camulodunum. Cassius Dio relates that he brought war elephants, although no remains of them have been discovered in Britain, and heavy armaments which would have overawed any remaining native resistance. Eleven tribes of South East Britain surrendered to Claudius and the Romans prepared to move further west and north. The Romans established their new capital at Camulodunum and Claudius returned to Rome to celebrate his victory. Caratacus escaped and would continue the resistance further west.
Late in 47 the new governor of Britain, Ostorius Scapula began a campaign against the tribes of modern day Wales, and the Cheshire Gap. The Silures of south east Wales caused considerable problems to Ostorius and fiercely defended the Welsh border country. Caratacus himself was defeated in one encounter and fled to the Roman client tribe of the Brigantes who occupied the Pennines. Their queen, Cartimandua was unable or unwilling to protect him however given her own truce with the Romans and handed him over to the invaders. Ostorius died and was replaced by Aulus Gallus who brought the Welsh borders under control but did not move further north or west, probably because Claudius was keen to avoid what he considered a difficult and drawn-out war for little material gain in the mountainous terrain of upland Britain. When Nero became emperor in AD 54, he seems to have decided to continue the invasion and appointed Quintus Veranius as governor, a man experienced in dealing with the troublesome hill tribes of Asia Minor. Veranius and his successor Gaius Suetonius Paulinus mounted a successful campaign across Wales, famously destroying the druidical centre at Mona or Anglesey in AD 60. Final occupation of Wales was postponed however when the rebellion of Boudica forced the Romans to return to the south east. The Silures were not finally conquered until circa AD 76 when Sextus Julius Frontinus' long campaign against them began to have success.
Agricola was recalled to Rome by Domitian and seemingly replaced with a series of ineffectual successors who were unable or unwilling to further subdue the far north. The fortress at Inchtuthil was dismantled before its completion and the other fortifications of the Gask Ridge in Perthshire erected to consolidate the Roman presence in Scotland in the aftermath of Mons Graupius were abandoned within the space of a few years. It is equally likely that the costs of a drawn-out war outweighed any economic or political benefit and it was more profitable to leave the Caledonians alone and only under de jure submission.
Later excursions into Scotland by the Romans were generally limited to the scouting expeditions of exploratores in the buffer zone that developed between the walls, trading contacts, bribes to purchase truces from the natives, and eventually the spread of Christianity. The degree to which the Romans interacted with the island of Hibernia is still unresolved amongst archaeologists in Ireland. The successes and failures of the Romans in subduing the peoples of Britain are still represented in the political geography of the British Isles today, with the modern border between Scotland and England running close to the line of Hadrian's Wall.