When the British intercepted the Zimmerman telegram, they realized Germany was offering a diplomatic proposal for Mexico to become their ally during World War I in case the United States were ever to take up arms against the Germans. However, this outraged the American public including the president at the time which was Woodrow Wilson, and this led to speeding up of American armament and the declaration of war on Germany.
Germany was ready to increase their war effort with renewed submarine warfare, and wanted Mexico's help to keep the United States out of the war in Europe.
Explanation:
As Germany decided to resume unrestricted submarine warfare in the Atlantic, the idea behind the telegram was that Mexico would engage in war with the United States, which would keep US forces and funding occupied on its own continent and prevent them from jumping into the war in Europe.
Here is the decoded text of the telegram sent by German Foreign Secretary Arthur Zimmermann to the German ambassador to Mexico, Heinrich von Eckardt:
<em>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 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.</em>
