Letters of Note is one of my favorite websites, and all the better when the letter in question relates to Mexico. To whit:
“Two years into World War I, on January 16th of 1917, the following coded telegram was sent by German Foreign Secretary Arthur Zimmermann (via the German Ambassador in Washington) to the German Ambassador in Mexico, Heinrich von Eckardt. In it, Zimmermann told Eckardt to approach the President of Mexico with a view to forming a military alliance between his country and Germany, in return for financial support and assistance in reclaiming Texas, New Mexico and Arizona from the US. En route to Mexico however, the telegram was intercepted by British intelligence, deciphered, and then cleverly leaked to the Americans, who at that point were neutral. Mass outrage followed, and just a couple of months later the US joined the war. ”
Read a translation of the original telegram.