Sussex emerged as a distinct entity, the territory of the South Saxons, in the 5th Century. Before that, there is archaeological evidence for hominid and early human settlement of this region of south east England. In historic times, the area was invaded and settled by various continental European peoples: the Atrebates from Gaul, the Romans and their Celtic clients, and the Saxons.
The Norman Conquest of England began with William landing his forces in Pevensey, Sussex and the Battle of Hastings took place in the county. Under the Normans and Angevins, Sussex was an important trade and military connection to their possessions in Normandy. With the loss of those possessions, Sussex was on the frontline of the Hundred Years War. Whilst ports in Sussex were incorporated into the confederation of the Cinque Ports, the only large harbours were at Chichester and Rye and the waters off the Sussex coast being difficult and the Sussex ports only serviced the local trade.
In medieval times, the county was known for its iron mining and working which only began to decline at the dawn of the industrial revolution. The arrival of the railways and the fashion for seaside bathing and holidays transformed Sussex from an isolated rural county of farms and small market towns.
The geography and size of Sussex meant that there were linguistic and other differences from east to west. With the Parliamentary Reform Act these divisions were formalised into an Eastern and a Western Division which became the basis of the county councils of East Sussex and West Sussex.