The correct answer to this open question is the following.
This message had the following effects: "if made the US decide to participate in World War 1. Basically, it pushed Woodrow Wilson to ask Congress for a declaration of war against Germany."
We are talking about the interception of the Zimmerman telegram. in which Germany asked México for its support in Worl War I. The US intelligence intercepted the telegram and this represented a major incident that made President Wilson enter the war. The other big incident was the sinking of the Lusitania ship. The United States entered World War I on April 6, 1917.