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.
The full story behind it is astonishing, and far too complex for the description above. I recommend starting at Wikipedia should you wish to learn more about what is one of the most important and fascinating telegrams in history.
Below is the telegram, its transcript, the transcript decoded, and, finally, the decoded transcipt translated to English.
Transcript
WESTERN UNION
TELEGRAMvia Galveston
130 13042 13401 8501 115 3528 416 17214 6491 11310
18147 18222 21560 10247 11518 23677 13605 3494 14936
98092 5905 11311 10392 10371 0302 21290 5161 39695
23571 17504 11269 18276 18101 0317 0228 17694 4473
23284 22200 19452 21589 67893 5569 13918 8958 12137
1333 4725 4458 5905 17166 13851 4458 17149 14471 6705
13850 12224 6929 14991 7382 15857 67893 14218 36477
5870 17553 67893 5870 5454 16102 15217 22801 17138
21001 17388 7446 23638 18222 6719 14331 15021 23845
3156 23552 22096 21604 4797 9497 22464 20855 4377
23610 18140 22260 5905 13347 20420 39689 13732 20667
6929 5275 18507 52262 1340 22049 13339 11265 22295
10439 14814 4178 6992 8784 7632 7357 6926 52262 11267
21100 21272 9346 9559 22464 15874 18502 18500 15857
2188 5376 7381 98092 16127 13486 9350 9220 76036 14219
5144 2831 17920 11347 17142 11264 7667 7762 15099 9110
10482 97556 3569 3670BERNSTORFF
Charge German Embassy.
Decoded Message
WESTERN UNION
TELEGRAMvia Galveston
Nr. 130, 13042, Auswaertiges Amt telegraphiert Januar 16: No. 1. Ganz geheim selbst zu entziffern. Wir beabsichtigen am ersten Februar uneingeschraenkt U-Boot Krieg zu beginnen. Es wird versucht werden, Vereinigte Staaten von Amerika trotzdem neutral zu erhalten. Fuer den Fall, dass dies nicht gelingen sollte, schlagen wir Mexiko auf folgend Grundlage Buendnis vor. Gemeinsam Krieg fuehren, Friedenschluss. Reichliche finanziell Unterstuetzung und Einverstaendnis unsererseits, dass Mexiko in Texas, Neu Mexico, Arizona frueher verloren Gebiet zurueck erobern. Regelung im einzelnen Euer Hochwohlgeborene ueberlassen. Sie wollen vorstehendes dem Praesident streng geheim eroeffnen, sobald Kriegsausbruch mit Vereinigten Staaten feststeht und Anregung hinzufuegen, Japan von sich aus zu sofortig Beitrachtung einzuladen und gleichzeitig zwischen uns und Japan zu vermitteln. Bitte den Praesident darauf hinweisen, dass ruecksichtlose Anwendung unserer U-Boote jetzt Aussicht bietet, England in wenigen Monaten zum Frieden zu zwingen. Empfang bestaetigen. Zimmermann.
BERNSTORFF
Charge German Embassy.
Decoded, Translated Message
WESTERN UNION
TELEGRAMvia Galveston
Nr. 130, 13042, Foreign Office has telegraphed on 16 January, No. 1. Strictly secret decode yourself. We intend to begin unrestricted U-boat warfare on February first. Effort will be made notwithstanding this to keep the United States neutral. In the event that we should not be successful in this, we propose alliance to Mexico upon the following basis: to make war together; make peace together; generous financial support; and agreement on our part that Mexico shall reconquer the formerly lost territory in Texas, New Mexico, Arizona. Arrangement of details to be left to your honor. You should disclose the foregoing to the President in strict secrecy as soon as outbreak of war with the United States is certain and add the proposal invite Japan to immediate spontaneous concurrent effort and at the same time use his good offices between us and Japan. Please call the President’s attention to the fact that the ruthless employment of our U-boats offers the prospect of forcing England in a few months to peace. Acknowledge receipt. Zimmermann.
BERNSTORFF
Charge German Embassy.