In January 1917 Mexico received a proposal from Germany. Germany proposed a military alliance with Mexico, through the form of a telegram. The content of the message was as follows (decrypted and translated into English):
- On the first of February we intend to begin unrestricted submarine warfare. In spite of this, it is our intention to endeavor to keep the United States of America neutral. In the event of this not succeeding, we propose an alliance on the following basis with Mexico: That we shall make war together and make peace together. We shall give generous financial support, and an understanding on our part that Mexico is to reconquer the lost territory in New Mexico, Texas, and Arizona. The details of settlement are left to you. You are instructed to inform the President [of Mexico] of the above in the greatest confidence as soon as it is certain that there will be an outbreak of war with the United States and suggest that the President, on his own initiative, invite Japan to immediate adherence with this plan; at the same time, offer to mediate between Japan and ourselves. Please call to the attention of the President that the ruthless employment of our submarines now offers the prospect of compelling England to make peace in a few months.
Germany had sent the telegram once, along official diplomatic channels connecting the Germany to the German ambassador in the USA, and from there to Mexico. They did not send a copy through diplomatic channels in Britain, as they did in our timeline. Mexican and German officials discreetly met in March, in Mexico City. After a week, they signed the Mexico-Germany pact, making the alliance official.
But while this was happening, German submarines had resumed unrestricted submarine warfare, and the United States declared war against them. Speed was of the essence; if Mexico could not mobilise quickly, the war would be over before they could get their lost territory back...
For more, see the following: