Mexico.
Step-by-step explanation:
The Zimmermann Telegram was a note that was sent from Germany to the United States but got intercepted by Great Britain.
Before the United States joined World War I, tensions were high between them and Germany. Germany was continuing to use unrestricted submarine warfare, despite America telling them several times to stop. President Woodrow Wilson did not want to join World War I and did not want to put his people through war.
Great Britain intercepted a note, now known as the Zimmermann Telegram, that was from Germany and going to Mexico. The note stated that if Mexico attacked the United States, they would give them land when they won the war. Obviously, the note never reached Mexico because it was intercepted, but that did not stop the rage within the United States.
The Zimmermann Telegram, along with Germany continuing to use unrestricted submarine warfare, caused the United States to join World War I. President Woodrow Wilson realized that he could not keep the country out of the war any longer and Germany was a real threat.