I listened to this book because I have kind of an interest in cryptography and its historical impact. The Zimmerman Telegram
is ostensibly about the famous telegram that was the final straw that brought America into the first World War, and how the British decoded it and then made use of it. But that turns out to be only a relatively minor part of the story. Really, most of the book is about the geopolitics of the early 20th century and the personalities of leading American, British, and German officials, diplomats, and military leaders, and how these shaped history as we know it.
The "plot" in a nutshell (and Barbara Tuchman does make this book interesting enough that it reads more like a novel plot moving from one twist to another, rather than the inevitable course of history): in 1917, Britain and the other Allied powers are getting the stuffing beaten out of them by Germany. The European front is hemorrhaging lives. What Britain wants and needs, and what Germany fears, is America entering the war. The only thing keeping Britain alive is her navy, and the German navy thinks they can starve Britain and the rest of the Allies if they commence "unrestricted" submarine warfare: meaning, even neutral ships are fair targets in the war zone. Since this largely means American ships bringing supplies to Britain, letting the U-boats loose means very likely provoking America into declaring war.
Then falls into the hands of British codebreakers, who unbeknownst to the Germans have broken their diplomatic code, a telegram from German Foreign Secretary Arthur Zimmerman to the German ambassador in Mexico. Zimmerman tells the ambassador to offer an alliance between Germany and Mexico should the U.S. enter the war (which they expect will happen since the decision has already been made to begin unrestricted submarine warfare). As part of the deal, Germany offers Mexico a great big slice of the American Southwest (basically everything the U.S. had taken from Mexico in various wars and then some), and also urges them to make an alliance with Japan to get Japan to attack the U.S. West Coast.
This is obviously political dynamite, and the British figure it's just what they need to push the U.S. into declaring war on Germany. The only problems are (1) how to reveal this in a way that will simultaneously not be dismissed by the Americans as a hoax while not revealing to the Germans that their code has been broken; (2) U.S. President Woodrow Wilson, who has been stubbornly persisting in trying to broker peace and keep the U.S. neutral, even as it becomes increasingly obvious that neither will be possible.
As I mentioned, the codebreaking stuff turns out to be a very small piece of the story. I found the characterization of President Wilson much more interesting: at times he seems naive, foolish, stubborn, and understandably his opponents even labeled him cowardly. He was adamantly opposed to entering the war, and was pushing his "peace without victory" plan even after the Germans had all but spit on it. But Tuchman's portrayal does suggest a man who was far from cowardly, and not a fool either. He genuinely wanted peace, and genuinely grieved when his orders resulted in the deaths of American servicemembers. (One might wish some of our more recent Presidents had such a personal investment in the consequences of their orders...) But he was also stubborn and prone to not listening to news and opinions he didn't like.
The other interesting part of the story is just how differently the U.S. was situated then as opposed to now. We Americans tend to think that the U.S. has been a "world power" pretty much since its founding, but really, in 1917, the U.S. was big and had a lot of industrial capacity and manpower, but had yet to really be tested on the world stage. Today we laugh at the idea that Mexico might seriously think they could invade the U.S. and carve off Texas, Arizona, and New Mexico, but it was no joke then, especially if Japan, a growing empire itself, landed troops on the West Coast, which was also a real possibility, or at least the U.S. believed it was.
World War I was when America had to actually prove itself and get bloodied. The other powers wanted America's strength on their side and feared America's strength turned against them, but probably no one had any idea of the global superpower the U.S. would become.
An interesting history full of diplomatic maneuverings and historical context that reminds us that everything leading up to World War I, like most wars, was built on things that had been happening for decades before it. A hundred years later, we mostly only remember the outcome.