On the morning of 7th December, 1941, the US Fleet suffered more damage in one hour than during the whole of the First World War, when Japanese aircraft bombed Pearl Harbour - the United States' largest naval base - and the airfields which were an essential part of its defences. The attack lasted from 7.55 am to 9.15 am, and initiated the Pacific Ocean as one of the war's major battlegrounds.
This distant theatre of war heralded a change in British submarine policy. As none of the existing submarines had adequate range, a new class - representing the only new submarine design produced by the Royal Navy during the Second World War - was adopted, which had a much greater range and increased surface speed. The opportunity was also taken to introduce major technical advances, of which the most important was an all-welded hull.
