In 1916 Woodrow Wilson won a second term as president, in part because he promised to keep America out of the growing war in Europe. Within months, Wilson—with the support of an overwhelming majority of Americans—would renege on that pledge. 

In January 1917, British code-breakers deciphered a secret telegram sent to Mexico by German Foreign Minister, Arthur Zimmermann. Zimmermann offered America's southern neighbor U.S. territory in exchange for joining Germany's side in World War I. When news of the telegram was published in American newspapers the outrage it inspired was swift and irrevocable. 

On April 6, 1917, the United States Congress formally declared war on Germany and its allies. The Zimmerman telegram clearly had helped draw the United States into the war and thus changed the course of history.

The Zimmermann Telegram
by Arthur Zimmermann

FROM 2nd from London # 5747.

"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 or 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 the 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 make peace." Signed, ZIMMERMANN.