From 58 BC to 51 BC, Caius Julius Caesar and his legions from Rome vanquished the Celtic civilization on the continent. He was not the leader of the Roman Republic at the time. He was the proconsul of Cisalpine Gaul and the Provincia Romana in what is now southern France, known at that time as Gaul. He needed to make a name for himself and increase his political status to help fulfil his ambitious quest for power. To do so required increasing his financial situation. Since most of the area he controlled had been looted by his predecessors, he looked to extend his domain northward. To this end he instigated what would become the Gallic Wars. (1)
As powerful as the legions were, it still took Caesar 8 years to conquer the Celts in Gaul and then he only did so by using a "divide and conquer" strategy. Though they were all Celts, spoke the same language and held similar beliefs, many Celtic tribes would fight against each other. Infringements of one tribe on another's land or livestock, a personal insult between tribes, or, basically, any human misunderstanding could set off a feud.
These feuds abetted the defeat of the Celtic clans of Gaul by Caesar. Some tribes fought alongside the legions against tribes they were feuding with while other tribes did nothing to interfere. This gave Caesar the opportunity to defeat the tribes one at a time. It was also the reason he had to destroy the druids and their religion.
Caesar's greatest fear of the Druids was that they would unite the clans. Their philosophical unity gave them great sway over the clans and they could have had an influence on them that would have been disastrous for Rome. Had the tribes united under a strong leader, the Romans might not have been victorious and the face of Europe might have changed drastically.
Approximately a hundred years after Caesar defeated the Celts in Gaul, in the year 43 AD. Claudius, emperor of Rome, ordered an invasion of Britain. Britain was still a Celtic stronghold at that time. His legions used the same divide and conquer strategy that Caesar used in Gaul. There were fourteen separate Celtic kingdoms in Britain. Again, the Romans feared that the Druids could unite the fourteen tribes so they systematically killed all of the priests. Even so, it took almost 40 years to subdue the British Celts (2)
The Roman legions that conquered Britain never defeated the Scots to the north. The Scots would raid Roman communities in the northern portion of England causing great commotion and loss of property and life. To stop the devastating raids by the Scots, the Romans built a wall across the island - it is known as Hadrian's Wall. For more information about Hadrian's Wall, visit their website at http://www.hadrians-wall.org/. (3)
Romans never conquered the west coast of Britain, particularly Cornwall and Wales, nor did they travel to Ireland. These areas have a distinct Celtic influence today. However, the remaining Druids in Britain and Ireland were converted to Christianity by the 4th century AD. In Ireland, the great intellectual interests of the Druids continued. Monastic orders were founded and monasteries were built throughout Ireland. The monks continued the tradition of gathering knowledge. Their record keeping and writing was not restricted to Christian religious topics. True, the scribes made copies of the bible, but they also recorded much of the information that had previously been the realm of the Druid priests. The Irish monks recorded many of the legends and stories of Celtic life and history.
Rome and most of Europe eventually fell to invasions from barbaric tribes such as the Vandals, Goths, Visigoths and, eventually the Huns. This brought the Dark Ages to the continent. The monks in Ireland and, in fact all of Ireland, were unaffected by the Dark Ages. Learning and intellectual pursuits never diminished as they did in the rest of Europe. (5) This is not to say that everyone on the continent became illiterate. Many nobles and early Church officials were educated, but the majority of the populace, who in earlier times had also been educated, was no longer.
The Dark Ages were pushed into oblivion by Irish monks during the sixth to eighth centuries AD when the monks returned to the continent to spread Christianity, bringing with them their accumulated knowledge and teachings. This would be the beginnings of a different type of conquest for the descendants of the Celts. The Irish monks brought their intellectual and religious teachings back to the continent. (6) They established monasteries, schools and libraries in Britain, France, Germany, Switzerland and Italy many of which exist to this day. (7)