Room 40

The Admiralty codebreaking section of the First World War

Overview

Room 40 was set up in the Admiralty Old Building in 1914 under Sir Alfred Ewing, with Captain William Reginald Hall, Director of Naval Intelligence from late that year, shaping how its product was used. Captured German naval codebooks, including one recovered by the Russians from the wrecked cruiser Magdeburg, let it read German naval signals and give warning of fleet sorties, including the one that led to the Battle of Jutland. Its most famous success was decrypting the Zimmermann Telegram in 1917, which helped bring the United States into the war. In 1919 it merged with army codebreakers to form the Government Code and Cypher School.