THE ZIMMERMANN TELEGRAM - Barbara Tuchman

In January 1917 the German Foreign Minister, Arthur Zimmermann, sent a cable to his Ambassador in Washington for transmission to the German Ambassador in Mexico.  In it, Zimmermann announced that Germany was going to start unrestricted submarine warfare on all shipping,Allied and neutral, in the Atlantic.  he then made a startling and audacious proposal: the German Ambassador was to offer the Mexican government an alliance directed against the United States and, furthermore, to ask the Mexicans to contact the Japanese to see if they could be persuaded to switch sides ( Japan was on the Allied side in the First World War).  British Intelligence decoded the telegram and passed it onto the Americans, with momentous consequences.

From the introduction by Margaret MacMillan

The momentous consequence was that this was the final catalyst that brought the United States into the First World War at a time when Britain was broke and was only months away from having to reach a settlement with the Germans.

This book backgrounds the German efforts to start a war between Mexico and the United States. Their thinking was if the US was tied up at home they would not have the inclination to fight in Europe even if the submarines were sinking tonnage of neutral shipping.

The US President, Woodrow Wilson , is shown to be so intellectually arrogant that all he could see was his vision for the world, not the reality of what was happening about him and he would not have come into the war without the translation of the telegram being supplied to him.

There is all the details of how the British with Russian assistance managed to obtain all the German codes which with some work enabled them to read all the German transmissions for the years of the war.

The British had one huge advantage which the Germans did not appreciate.  British cable ships had cut all the German transatlantic cables at the start of the war ( day two actually).  The Germans had to use British controlled cables or wireless but they were not duly alarmed because they were convinced that their codes could not be broken.
  
From the introduction by Margaret MacMillan.

This is fascinating from start to finish, its a short read ,170 pages, but reads like a thriller.  Great stuff.