Decryption of what, was celebrated as a significant, game-changing intelligence triumph for Britain during World War I?

Question

World War I was triggered as a full-blown war when the United Kingdom declared war on Germany on 4 August 1914, when Germany invaded which country?

Answer 1

A telegram from the German Foreign Secretary to the German ambassador to Mexico
The 1917 Zimmermann telegram proposed a military alliance between Germany and Mexico in the event of the United States' entering World War I against Germany, and an offer of American territory to Mexico in exchange. The revelation of the telegram stirred enough antagonism in the USA finally to prompt the USA to enter the war.