(The impetus for this post is my recent reading of a manuscript of Thomas Boghardt’s The Zimmermann Telegram, forthcoming from the Naval Institute. Thomas himself will be making a guest appearance in Part II).
Henry Stimson abolished America’s codebreaking office, famously saying that “Gentlemen do not read each other’s mail.” He was invoking, as David Kahn pointed out, not diplomatic niceties but moral ones:
[Stimson] did not say ‘Diplomats do not read each other’s mail but ‘Gentlemen do not read each other’s mail.’ For actually his point was larger. Reading another’s mail was theft and therefore wrong, not just for diplomats, but for everybody. Gentleman exemplify man’s moral obligations. At the root of Stimson’s observation lay not a legalism but a Commandment: Thou shalt not steal.
Apparently, dropping the atomic bomb was something that gentlemen did, but that’s a discussion for another day.
Also a discussion for another day is the article I encountered while researching Stimson’s quote, which started off with:
‘Gentleman do not read each other’s mail.’ This was Secretary of State Henry Stimson’s post-Pearl Harbor justification for closing the State Department’s code-breaking office in 1929.
WHAT ARE THEY TEACHING IN THE SCHOOLS THESE DAYS!?
In any case, what this post is really about is the Zimmermann Telegram. I know, that’s a bit of a shift, but the connection will become clear. The Zimmermann Telegram was the German message, sent during World War I, that essentially attempted to bring Germany and Mexico together in an alliance against the United States. Germany’s campaign of unrestricted submarine warfare looked likely to bring the US into the war, and German diplomats thought that having Mexico on their side would help distract the Americans.
This was not particularly a good idea. It became an even worse one when the British intercepted the telegram, decrypted it, and sent it to the Americans, causing an outburst of outrage and hurrying American intervention.
Copies of the decrypted telegram are here.
The story, as the British told it, was that a British agent working in Mexico had managed to acquire a copy of the telegram and the British had decrypted it and passed it on to the U.S.
The real story was much more complicated. The British were actually spying on American transatlantic cables. Apparently, either gentlemen did read each other’s mail, or the British weren’t gentlemen. In any case, the British had started intercepting American diplomatic traffic where the American cable from Denmark to the US touched land in Britain for a signal boost. At the start of the war, similar German cables had been cut by the British, leaving the Germans without a method of communicating with their embassy in Washington. In the interests of keeping open such communication the (then-neutral) United States agreed to allow the Germans to send a certain number of communications on the American cable, including eventually Zimmermann’s missive to the Mexicans.
The cheek of sending a proposal for an alliance against the United States over US wires is duly noted, but the really appalling embarrassment was that the Germans should have known that the British had physical access to the US cable. Given that, they should have at least contemplated the idea that the British were reading everyone’s mail. But they didn’t, much to their dismay.
[Part II on Monday]
 David Kahn, The Reader of Gentlemen’s Mail : Herbert O. Yardley and the Birth of American Codebreaking (New Haven: Yale University Press, 2004), 101.