Information from the following sources was used for this section.
Peter Freeman (2006) The Zimmermann Telegram Revisited: A Reconciliation of the Primary Sources, Cryptologia, 30:2, 98-150, DOI: 10.1080/01611190500428634
Joachim von zur Gathen (2007) Zimmermann Telegram: The Original Draft, Cryptologia, 31:1, 2-37, DOI: 10.1080/01611190600921165
The Zimmermann Telegram: America Enters the War, 1917-1918 (1966) – Tuchman, Barbara
The Zimmermann Telegram of January 16, 1917 and its Cryptographic Background – Friedman, William F. and Mendelsohn, Charles J. – War Department