Suppose the UK hadn't been able to decode the Zimmerman Telegram in 1917?
Wily,
First, check out David Kahn's
The Codebreakers. It's chapter on Room 40 contains more information regarding the Zimmerman Telegram, it's transmission, decryption, eventual use, and fallout, than I've have found in any other single volume.
Second, if Room 40 hadn't been able to decode the Telegram there would have been far greater effects on the course of WW1 than just the US' entry into that war. By 1917 the UK was essentially reading any German radio and cable traffic it cared to, so British actions and policy reflected that capability. If Room 40 were unable to decode the Zimmerman Telegram that would also mean that Room 40 would be unable to decode other German communications. When you remember that during the run-up to Jutland and thanks to codebreaking and signals intelligence the Grand Fleet left Scapa
before the High Seas Fleet left the Jade, you can see the many possibilities and/or problems that lack of codebreaking would create.
Third, Mexico wasn't just "likely" to say
Oh, heck no!" to the proposal in the Zimmerman Telegram. When the contents of the Telegram were revealed Mexico took extreme care to repudiate as strongly as possible and as often as possible any suggestion that they'd even considered the proposal. Given Mexico's condition at the time and given the fact that the US was in the middle of yet another major interventionist period in Latin America, even contemplating about aligning themselves with Germany in order to somehow recover Texas, New Mexico, Arizona, and California would have been
suicide(1). In fact so dangerous was the Telegram to Mexico, Mexican authorities actually assisted agents of the UK in stealing a copy of the Telegram from a cable office in Mexico City, so that the UK could claim they hadn't originally intercepted the Telegram on a US cable.
There is no chance - NO, as in NONE, as in NEVER - of Mexico accepting the proposals within the Zimmerman Telegram without there also being so many changes to the OTL as to make the conditions that led to the Telegram being sent wholly implausible.
This is a non-starter.
Bill
1 - Despite what US schools still teach and too many people still believe, sinkings of neutral vessels by German submarines
did not lead to a US declaration of war. Despite years of sinkings, the nation was basically split on the issue as late early 1918 with the East wanting to enter the war while the South and West wanted to stay out. It was only when Germany proposed to somehow return CA, AZ, NM, and TX to Mexico that national public opinion changed. The Telegram was released on March 1st and the US was at war on April 6th.