A encoded telegram was sent from the Foreign Secretary of the German Empire Arthur Zimmermann to the German ambassador to Mexico, Heinrich von Eckardt reading as follows..
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.
The text proposed to invite Mexico into World War One as a Germany ally with the enticement that it was to receive those territories lost during the Mexican War. Rather obviously Germany lacked a concrete understanding as to the degree of Mexican military strength, but as absurd as it sounds, in 1915 some vague Mexican revolutionary forces actually considered, and indeed attempted, to sponsor an uprising in that territory, albeit to little effect. And Carranza's government did study the proposal, finding it unrealistic.
The note was decoded by the British in subsequent days, as will be seen, with negative consequences for Germany