What the title gets wrong
The Roman Empire was not conquered as a whole by any outside power. That is the first thing to say about this subject, because the title suggests a single conquest. The empire was the conqueror, not the conquered. Its history ended in two quite different ways. In the west, the imperial government faded in the fifth century CE, and 476 CE is the date conventionally used for the end, when the Germanic commander Odoacer deposed the last western emperor, Romulus Augustulus. In the east, the empire continued, now commonly called the Byzantine Empire, though its people called themselves Romans, until the Ottoman sultan Mehmed II captured Constantinople in 1453.
So there are two stories under this heading: Rome’s own conquests, which built the empire, and the different processes by which it ended. This article covers the first in some detail and the second in outline.
The conquest of Gaul
The most famous Roman conquest is probably that of Gaul, the lands roughly corresponding to modern France, Belgium and parts of neighbouring countries. Julius Caesar, as governor, fought there between 58 and 50 BCE. Much of what we know comes from his own account, the Commentaries on the Gallic War, which is a valuable source but also a political document written to present him favourably. Its claims about numbers and casualties should be treated with caution.
The Gauls were not a single state. The peoples of the region, often described together as the Iron Age Celtic tribes, were divided, and Caesar made use of their rivalries. In 52 BCE a large revolt under Vercingetorix, a leader of the Arverni, united many of them. The turning point was the siege of Alesia, where Roman forces built lines of fortifications facing both inward and outward, trapping the defenders while holding off a relief army. Vercingetorix surrendered. Gaul was organised into provinces under Augustus, and over the following centuries it became deeply Romanised. Caesar’s army also relied on skilled camp-building and siege methods like those described in the article on Roman marching camps.
Contemporaries and later scholars still argue about the human cost of the campaigns. Ancient figures are large, and are not reliable enough to quote as exact numbers.
Britain
Caesar crossed to Britain in 55 and 54 BCE, but these were raids and did not lead to occupation. The real conquest started in 43 CE under the emperor Claudius. Roman forces gradually took over the south and east, and the new province was governed from a base at what became Londinium. A major revolt led by Boudica of the Iceni broke out around 60 or 61 CE and destroyed several towns before it was suppressed.
Rome advanced north and west but never conquered the whole island. In the north, a succession of campaigns reached into present-day Scotland, but the Romans pulled back to a frontier line, with Hadrian’s Wall begun in the 120s CE and a later wall further north under Antoninus Pius. The story of the peoples beyond the frontier is connected with the matter of the Picts and Romans, although the name Picts is recorded only later. Roman rule in Britain ended in the early fifth century CE, when the imperial government ceased to administer it directly.
Dacia
Dacia, north of the lower Danube, was a kingdom with strong fortifications and its own culture. After earlier clashes with Rome, the emperor Trajan fought two wars against the Dacian king Decebalus, in 101-102 and 105-106 CE. Decebalus died in the second war, and the territory became a province. Gold and other resources there are often mentioned among the reasons for the conquest, though the exact motives are debated.
Dacia was one of the last major provinces added in Europe. It was abandoned in the 270s CE under pressure from migrations and warfare, and the troops and some civilians were withdrawn south of the Danube. Its story shows that Roman expansion could be reversed. The background to the region appears in the profile of the Dacian kingdom.
Other conquests and the limits of expansion
Rome also conquered Spain over a long period, Egypt in 30 BCE and many other lands. Its growth was not unlimited. Augustus tried to extend the frontier to the Elbe, but a disaster in the Teutoburg Forest in 9 CE, in which three legions were destroyed, ended that plan, and the Rhine and Danube became the long-term boundary in much of Europe. Under Trajan the empire briefly pushed into Mesopotamia, but his successor withdrew from these gains. Territories annexed later included the Nabataean kingdom, a step covered under the Roman annexation of Nabataea.
The western end, 476 CE
By the fifth century CE the western empire faced pressure from several groups settling or invading within its borders, along with internal rebellions, financial strain and weak government. Historians debate how much each factor mattered. Rome itself was sacked in 410 and again in 455, but neither event was the end. The year 476 is conventional because Odoacer sent the imperial regalia to Constantinople and ruled Italy in his own right. Contemporaries do not seem to have seen it as a clear break, and many institutions continued. The longer story is told in the study of the fall of Rome.
The eastern end, 1453
The eastern empire survived for another thousand years. It lost territory to various rivals, was weakened by the Fourth Crusade’s capture of Constantinople in 1204, and gradually shrank. By the fifteenth century it consisted of the city and little more. Mehmed II’s capture of Constantinople in 1453 is usually taken as its end. This was a conquest of a surviving Roman state, but by then it was a small remnant, not the empire of Caesar and Trajan.
Putting it together
Roman conquest of Gaul, Britain and Dacia was real and well documented, though the sources are partial. A conquest of the Roman Empire as a whole never took place: the western half gave way to successor kingdoms in the fifth century, and the eastern half lasted until 1453.