At the start of World war 2 the world war 1 defences were brought back into use and further blockships sunk. But they did not work. On 14 October 1939, The German U-Boat, U-47, took advantage of a high tide and got past the blockships and into Scapa Flow. Once there, U-47 torpedoed HMS Royal Oak and left the same way it had entered. Over 800 members of the Royal Oak's crew were killed.
Soon after this had happened Winston Churchill visited Orkney and ordered that work should begin on the construction of four permanent barriers linking together the 4 small islands together South Ronaldsay, Burry, work began began for this in May 1940.
The Churchill Barriers were formally opened by the first Lord of the Admiralty on 12 May 1945 however just as they were oppenend the war's ended. As a result their lasting role was not as a defence for Scapa Flow, but as a series of causeways linking the five islands together.