Germany also wrote a secret telegram note to Mexico in code suggesting that the two countries work together to attack the United States. This note is called the Zimmerman Telegram because it was sent by Arthur Zimmerman. It offered Mexico land in the southwestern United States that the United States took in previous wars. Spies from the United Kingdom found out about the note and told the United States. American people became angry and many decided that they wanted their country to enter the war against Germany. For the Zimmermann Telegram as well as the sinking of American ships by German U-boats, on April 6, 1917 the United States declared war against Germany and joined the Allies.