According to the National Archives:
“In January of 1917, British cryptographers deciphered a telegram from German Foreign Minister Arthur Zimmermann to the German Minister to Mexico, von Eckhardt, offering United States territory to Mexico in return for joining the German cause.”
The telegram was decoded to read:”We intend to begin on the first of February
unrestricted submarine warfare. We shall endeavor in spite of this to keep the United States of America neutral. In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis:make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer lost territory in Texas, New Mexico, and Arizona. The settlement in detail is left to you. You will inform the President of the above most secretly as soon as the outbreak of war with the United States of America is certain and add the suggestion that he should, on his own initiative, invite Japan to immediate adherence and at the same time mediate between Japan and ourselves. Please call the President’s attention to the fact that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to peace.”
The American public found out about the telegram when it was printed in the newspapers on March 1, 1917 after President Wilson authorized its release.The American public, already upset over the sinking of the Lusitania in 1915 by a German submarine, was incensed.
The public outcry this telegram incited is often seen as the reason the United States finally entered the war.
Want to know more? Check out:
The Zimmermann Telegram by Barbara Tuchman, Read by Wanda McCaddon (Army Overdrive)
or one of the libraries’ great databases for articles like:
“Lucky break: David Nicholas reveals the skill and good fortune behind Britain’s First World War intelligence operation, and the coup by which the Zimmerman Telegram was cracked, tipping the balance in getting the US to join the Allied war effort” From History Today, September 2007 (link requires Army MWR Library account)