The Zimmerman Telegram. Barbara Tuchman. Papermac (1958)

This is the extraordinary story behind the event that finally propelled the USA into the First World War. The event was a telegram from the German Foreign Secretary to the German Ambassador in Mexico sent via the German Ambassador in the USA on the17 January 1917. The telegram announced that unrestricted submarine warfare would be recommenced and much more significantly Germany would support and attack by Mexico on the USA. Barbara Tuchman provides both the wider context for the plans announced in the telegram and the impact it had as well as the amazing story of how it was intercepted, decoded and finally revealed.
At the heart of the story about how the telegram was found and used is the essential problem that any spying activity has to confront, how to use the information that has been discovered without revealing the process used to uncover it. This was particularly acute in this case as the British had cracked the German codes early in the war and the Germans never knew and this provided a steady stream on critical intelligence. Any risk to this had to avoided, yet the information in the telegram was recognised as the key to getting the USA into the war which was the only chance the Allies had to survive let alone win the war.
The German plan was based on both sound strategy and wishful thinking, unrestricted submarine warfare would quickly and efficiently bring England to economic ruin and military standstill. This strategy was recognised as being very likely to bring the USA into the war, the wishful thinking was that a domestic war front could be opened with Mexico that would distract the USA from Europe. As Barbara Tuchman makes clear it was not an entirely implausible plan, it fatally misjudged the situation due to the overwhelming need for it to be true.
The espionage aspect to this story is beyond the wildest realms of spy fiction, fiction is constrained by the need to be credible and the actual events are absurd in the extreme. In particular the events surrounding the acquiring of a releasable version of the telegram in Mexico are jaw dropping.
The whole book is superbly written, the gripping story in given force and clarity and the whole context of the war provided in just the required level of detail. Unmissable.