<P> During World War I, on the eve of the Battle of the Somme on 30 June 1916, the Royal Sussex Regiment took part in the Battle of the Boar's Head at Richebourg - l'Avoué . The day subsequently became known as The Day Sussex Died . Over a period of less than five hours the 17 officers and 349 men were killed, including 12 sets of brothers, including three from one family . A further 1,000 men were wounded or taken prisoner . </P> <P> With the declaration of the World War II, Sussex found itself part of the country's frontline with its airfields playing a key role in the Battle of Britain and with its towns being some of the most frequently bombed . As the Sussex regiments served overseas, the defence of the county was undertaken by units of the Home Guard with help from the First Canadian Army . During the lead up to the D - Day landings, the people of Sussex were witness to the buildup of military personnel and materials, including the assembly of landing crafts and construction of Mulberry harbours off the county's coast . </P> <P> Since its creation in the fifth century, Sussex has been subject to periodic reform of its local governance . After the Reform Act of 1832 Sussex was divided into the eastern division and the western division, these divisions were coterminous with the two archdeaconries of Chichester and Lewes . In 1889, following the Local Government Act 1888, using those same boundaries, Sussex was divided into two administrative counties, East Sussex and West Sussex together with three self - governing county boroughs, Brighton, Eastbourne and Hastings . </P> <P> In the post-war era, the New Towns Act 1946 designated Crawley as the site of a new town . </P>

When was sussex split into east and west