In what year was the diplomatic letter held by Arthur von Zimmerman retrieved by British Intelligence? (WW I)?

Question

(A diplomat of Germany) Arthur von Zimmerman's letter concerned an alliance with Mexico and submarine warfare with the United States. Any research sites with the details of this event would be helpful to my research.

Answer 1

1917. You can read about the whole affair in a paperback book, "The Zimmerman Telegram," by Barbara Tuchman. Try amazon.com or alibris.com.