The first world war admiral, ‘Blinker’ Hall — so-called for the obvious reason — is less widely known than Jellicoe, Beatty & Co., but his contribution to victory and history was arguably greater. He was the Director of Naval Intelligence (DNI) who ensured the success of Room 40, the 1914 equivalent of Bletchley Park in 1939. Less famous than its successor, partly because radio was less used then, its ability to decrypt German naval and diplomatic ciphers was no less significant. Not the least of its achievements, enhanced by Hall’s outstanding political skill, was the decoding of the Zimmerman telegram, which effectively brought America into the war.

Hall was born into a naval family in 1870 (his father was the first DNI) and, with early command and the patronage of Beatty, seemed destined for stardom until a weak chest cut short his fighting career after the Battle of Heligoland Bight in August 1914. When he took over as DNI, the fledgling Room 40 was in danger of being smothered by reams of intercepted German radio cipher traffic, all unreadable. Many, including Churchill, believed the new technology could never be deciphered.

But Room 40 benefited from three early strokes of luck. In the Baltic the Russians seized codebooks from a sinking German cruiser and gave them to Churchill; in an action off the Dutch coast the Navy sank four German destroyers, from one of which a trawler subsequently dredged up a chest containing further codebooks; meanwhile, the Royal Australian Navy had captured a German steamer that was unaware that war had started and from it seized a cipher key that made sense of the codes. It wasn’t only luck, though: shortly after midnight on the day war was declared the British navy cut the German undersea cables carrying all their signals traffic to Spain and across the Atlantic, thus forcing them to use radio.

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