In 1917, British interception and decoding of a German diplomatic Telegram helped bring the United States into the First World War.

At the outset of the First World War, the British navy cut Germany’s five trans-Atlantic cables and six underwater cables which ran between Britain and Germany. The British then tapped into overseas cable lines that Germany borrowed from neutral countries to send communications. As a result, Britain began capturing large volumes of coded German communications.

The Cryptanalysis section of British naval intelligence (known as Room 40) worked over the course of the war to decipher these communications. Thanks to a stolen copy of a German diplomatic code gained in the near east and the Russian admiralty sharing a copy of the German naval codebook it had gained, by 1917 British Intelligence could decipher most German messages.

In January of 1917 the German Foreign Secretary, Arthur Zimmerman sent a coded telegram to Johann Von Bernstorff, the German Ambassador in Washington. The telegram stated that if unrestricted submarine warfare threatened to bring America into the war, then he should approach Mexico to enter the war on Germany’s side.

The telegram was picked up and decoded by Room 40’s cryptologists. This caused a dilemma as Britain needed to find a way of informing the Americans of their discovery without the Germans realising their codes had been broken and without the U.S. realising communications to their country was being read by British intelligence. To solve this problem, Britain handed over a decoded version sent via the German Embassy in Washington to Mexico, so that it would appear that the document had been leaked in Mexico instead.

President Wilson released the contents to American newspapers and it quickly led to a public uproar in across the country. Then on 4 April 1917 America Declared War on Germany.