Although such a change of policy would cut off supply lines and possibly starve Britain into submission within six months, there was a risk attached to this strategy. Up until this point, President Woodrow Wilson had kept America neutral, but the inevitability of civilian casualties resulting from all-out U-boat aggression threatened to draw America into the war. Consequently, the German Foreign Minister, Arthur Zimmermann, decided to draw up an insurance policy.
Zimmermann sent a telegram to the President of Mexico, stating that, in the case of America's entering the war, then Germany would support a Mexican invasion of America, helping it to reclaim territories such as Texas, New Mexico and Arizona. If Zimmermann's plan worked, then America would be too busy defending itself at home to become involved in the European conflict. Ideally he would have sent the message via Germany's own transatlantic cables, but, before dawn on the first day of the war, the British ship Telconia had approached the German coast under cover of darkness and severed Germany's transatlantic cables. This act of sabotage forced Zimmermann to send his telegram via cables that touched Britain.
Zimmermann had encrypted the telegram, and assumed that the German codes were strong enough to protect his message, but he underestimated the skills of the British codebreakers, who immediately set to work deciphering the telegram. The Admiralty's codebreaking office, known as Room 40, were well versed in cracking a whole variety of codes. For example, Room 40 had solved the mystery behind a Turkish postcard that had been addressed to Sir Henry Jones, 184 King's Road, Tighnabruaich, Scotland. Sir Henry assumed that it was from his son, a prisoner of the Turks, but he was puzzled because the postcard was blank, and the address was peculiar - none of the houses in Tighnabruaich were numbered and there was no King's Road.
Room 40's codebreakers realised the address alluded to the Bible, I Kings, chapter xviii, verse 4: "Obadiah took a hundred prophets, and hid them fifty in a cave, and fed them with bread and water." Sir Henry's son was simply reassuring his family that he was being well looked after by his captors.
Although Room 40 rapidly deciphered the Zimmermann telegram, the British did not immediately show it to the Americans. If the Americans entered the war because of the hostile contents of the telegram, then the Germans would realise that their diplomatic code had been broken, and they might then upgrade their codes, depriving the British of a valuable source of intelligence. Furthermore, unrestricted U-boat warfare was due to begin in just a matter of days, and this in itself might provoke the Americans into entering war. Why risk losing a source of intelligence, if the Americans might already be on the verge of joining the Allies?
On 1 February, Germany embarked on its strategy of unrestricted U-boat warfare, but two days later President Wilson announced to Congress that America would continue to remain neutral. This left the British with no choice but to reveal the contents of the Zimmermann telegram.
At the beginning of the year Wilson had said that it would be a "crime against civilisation" to lead his nation to war, but the Zimmermann telegram forced him to change his mind: "I advise that the Congress declare the recent course of the Imperial Government to be in fact nothing less than war against the government and people of the United States, and that it formally accept the status of belligerent which has thus been thrust upon it." A single breakthrough by Room 40's codebreakers had succeeded where three years of intensive diplomacy had failed.
Simon Singh is the author of `The Code Book - the science of secrecy from ancient Egypt to quantum cryptography' (Fourth Estate, pounds 16.99)