d) Zimmerman Telegram revelation

The revelation of the Zimmerman Telegram was a significant factor in the U.S. decision to enter World War I. This secret diplomatic communication from Germany to Mexico proposed a military alliance between the two nations, should the U.S. enter the war against Germany. The British interception and decryption of the telegram, and its subsequent publication, outraged the American public and pushed the United States closer to declaring war on Germany, which it did in April 1917.