In 1917, the United States became aware of a telegram that had been sent by the government of Germany to that of Mexico. The telegram had requested that, in order to prevent the United States from becoming involved in Germany's European war, Mexican forces invade the United States, with the promise of land in the south of the country in the event of a German victory in Europe.
This telegram prompted the President of the United States, Woodrow Wilson, to declare war on Germany and throw the full military and industrial strength of the United States behind the European Allies, the nations that Germany was at war with, and is widely believed to be a major factor leading to the Allies' victory the following year.
For Want of a Telegram imagines a timeline in which the telegram was never sent, nor even conceived of. The idea was simply never had. As a result, although the US continues to fund and arm the Allies, American manpower never comes into play. This means that, following the capitulation of the Russian Empire and the massive shift in German and Austro-Hungarian resources to the west, there is no American counterbalance and the Allies ultimately lose the war.
The absence of the telegram is this timeline's most significant PoD, but it is not the only one. An earlier one occurs at the outbreak of war, when Italy and the Netherlands both join the Central Powers in the war against the Allies - Italy to honour its alliance with Germany and Austria-Hungary, the Netherlands under duress. The involvement of these two nations radically alters the course of the war and the map of the world at its conclusion.