Invasions of the British Isles have occurred throughout history. Indeed, various states within the territorial space that now constitutes the British Isles were invaded several times; by the Romans, the Germans, Scandinavians, the French, the Dutch and one another.
This page currently covers invasions from 2000 BC only up to the Glorious Revolution (or called the bloodless revolution) in 1688.
Before the Romans came to Britain, and with them the advent of written records of the region, the majority of Britain was Celtic. How and when these peoples arrived in the British Isles is a matter of much conjecture; see Celtic settlement of Great Britain and Ireland for more details.
In 55 BC, Celtic Britain was invaded by the Romans under Julius Caesar. Caesar's two invasions did not conquer Britain, but established it as a major trading partner of Rome.
A century later, a botched attempt to conquer Britain was made under the emperor Caligula. Caligula's uncle and successor, Claudius, was the first emperor to oversee a successful invasion. He used as an excuse the pleas for help that came from the Atrebates, Celtic allies of Rome, and landed an army near present-day Richborough. The initial landings were unopposed, and the Celts delayed in responding to the invasion. When, under their leaders Caratacus and Togodumnus, they did, they were too late and were defeated in several battles, most notably that of the River Medway.
Claudius arrived himself, bringing up to 38 war elephants with him. When the Celts were finally defeated and Caratacus forced to flee to Wales, Claudius returned to Rome.
In the early AD 60s, the Celtic tribal queen Boudicca led a bloody revolt against Roman rule. While the governor Suetonius was pursuing a campaign on the isle of Anglesey, Boudicca, angered by maltreatment at the hands of the Romans, urged her people to rise up. They did, and marched on Camulodonum (now Colchester), where many former Roman soldiers had settled.