Why was the Zimmerman Note significant to WWI?
The Zimmermann Note, or telegram, was sent by German Foreign Minister Arthur Zimmermann in 1917. He had sent it to his ambassador in Mexico and the goal was to get Mexico to become an ally of Germany. He offered a great deal of financial support along with Texas, Arizona, and New Mexico if they won the war. The note also encouraged Mexico to gain Japan as an ally as well.
President Wilson received the note and it was released to the the United States public and they were outraged. Mexico and Japan denied any involvement with Germany. The United States declared war soon after.
The Zimmermann Telegram, or Note, was significant to the history of World War I because it forced United States President Woodrow Wilson to reverse his initial position on American involvement in the European conflict and commit the United States to the war against Germany.
Named for German Foreign Minister Arthur Zimmermann, from whose ministry the telegram was dispatched, the message was directed to Germany's ambassador to Mexico, Heinrich von Eckhardt. The substance of the message (the decoded and translated text of which is available at the link provided below) was the proposition of an alliance between Germany and Mexico for the purpose of destabilizing the United States and, consequently, preventing American attention from being able to focus on the conflict raging across the Atlantic. Mexico, according to the Germans, would receive territory lost to the United States in exchange for its declaration of war against its northern neighbor. In addition, the telegram suggested that Germany's practice of carrying out unrestricted submarine warfare against "enemy" shipping--in effect, the commercial as well as military maritime transport of England, France, and other German adversaries--would resume following an earlier cessation.
The Zimmermann Telegram was brought to President Wilson's attention by the British Government, whose intelligence service had intercepted the transmission and decoded it. Britain, a major combatant in the war, had a very real stake in encouraging American involvement in the conflict on its side, and President Wilson's reaction to it served that purpose.
The Zimmermann Note (it has two "ns" at the end -- I never realized that until I taught a German exchange student) was a major factor in getting the US to enter WWI.
This note was sent from Germany to Mexico. It proposed to have Mexico enter the war against the US, on the side of Germany. It said that Germany would, after it won the war, give the Southwestern US back to Mexico.
This helped get the US into the war by inflaming US public opinion. When the note was leaked (the British had intercepted it) it mad Americans quite angry.