Gradual Annexation and Roman Rule
During the period 43 to 410 AD, the southern half of what is now the British Isles, consisting of England, Wales, and parts of Scotland, was a de facto Roman colony, referred to as Britanniae.
Southern Britain had already been invaded by Julius Ceasar almost a century before the Roman domination began, and several invasions took place since. It was finally the emperor Claudius who formally annexed the first parts of Britain to the Roman empire.
The Romans annexed Britain little by little and with great difficulty. There was often formidable opposition from the side of the local populations to the colonization. The Roman advance was often on hold due to British revolts, most famously by the warrior queen Boudica.
The advance finally stopped in southern Scotland, which the Romans never managed to conquer. The southern colony was often subject to barbarian invasions until the Romans were eventually forced to withdraw.
Precursor to Medieval England
Roman influence on Britain decreased on par with its political and military power in continental Europe. Britain was always under siege from the north by barbarians who were never conquered. Finally, the capital no longer could provide forces to defend the colony, and it was gradually lost.
Romans contributed significantly to the future development of England’s politics, trade, and society. They expanded the infrastructure by building roads and developing agriculture and introduced new rigorous methods of administration. They also contributed to the culture and philosophy.
So profound was the Roman influence that even after their withdrawal, the re-emergence of local chiefs as a political force, Roman culture and values were visible in politics. Britain also became a more influential economic and commercial power, extending its trade routes well beyond its imperial masters.
In the centuries following the Roman rule, a profound transformation took place in Britain, with Anglo-Saxon culture becoming predominant. In many senses, this marks the emergence of England as a nation.