In the early hours of 5 August 1914, only a few hours after war was declared, Britain carried out something that seemed to be minor, but was actually vital. A British cable ship severed five German overseas underwater cables, which passed from Emden through the English Channel to Vigo, Tenerife, the Azores and the USA
This cut direct German communications to outside Europe, most significantly to the United States. The British could now intercept German signals to their embassies. They were sent in code, but British codebreakers were eventually able to read them.
Most significantly, Britain intercepted the Zimmerman Telegram, sent to the German Ambassador to Mexico. If the USA went to war with Germany, he was to offer the Mexicans an alliance with the promise that they would receive Texas, New Mexico and Arizona. Its revelation helped to push the USA into war with Germany.