The Zimmermann Telegram: America Enters the War, 1917-1918; Barbara W. Tuchman's Great War Series

In January of 1917, the war in Europe was, at best, a tragic standoff. Britain knew that Europe could be saved only if the United States joined the war. But President Wilson was unshakable in his neutrality and in his efforts to mediate peace. Then, with a single stroke, the tool to propel the United States into World War I came into a quiet British office. One of countless messages intercepted and read by the crack team of British decoders in room 40, the Zimmermann telegram was a topsecret message to the President of Mexico, inviting Mexico to join Germany and Japan in an invasion of the United States. Mexico's reward: recovery of her lost American territories. Germany's goal: to keep American fully occupied on her side of the Atlantic.
How Britain managed to inform the United States of Germany's plan without revealing that the German codes had been broken makes for an incredible, true story of espionage and intrigue as only Barbara W. Tuchman could tell it.