The assassination of the heir apparent to the Austro-Hungarian Empire in Sarajevo (Bosnia) by a Serbian nationalist was the immediate cause of World War I. Though the great European powers had good reason to resolve the conflict between the Austro-Hungarians and the Serbs peacefully, mutual suspicions and fears of being put at a military disadvantage led to a widespread mobilization that escalated out of control. Two blocs of power emerged: the Entente or Allied Powers (Britain, France, Russia, and later Italy) and the Central Powers (the German and Austro-Hungarian Empires). Both sides expected the war to be short and the outcome to be determined by one or two decisive battles.

This war was first known as "The Great War." Only after 1939 did the term World War I come into common usage. Regardless of the name, the Great War was not simply a European conflict. Indeed, precisely because the combatants were also imperial and colonial powers, the war quickly took on a global dimension and eventually involved the United States.

American involvement was a consequence of several factors. The German policy of unrestricted submarine (U-Boat) warfare against all shipping in British waters, even the shipping of neutral countries including the United States, was unpopular among Americans and contributed significantly to the eventual decision to declare war on Germany. British propaganda, moreover, successfully depicted Germany as an evil aggressor, stressing Prussian atrocities against "little Belgium" and the horrors of the submarine campaign. Also, American financial and industrial concerns profited mightily by servicing British and French wartime needs; a defeat of Britain and France and the resulting default on their loans would have had disastrous consequences on U.S. financial markets.

This module is divided into five parts. The first provides an overview of the conflict on the western front in 1914 . The second section continues the focus on the western front, from 1915 to 1918 . The War in Eastern Europe and the Middle East overviews these "fronts." The Colonial War deals with the worldwide dimension of the war, especially the dismemberment of the German Empire. The final section chronicles the shift in the United States from neutrality to belligerency.

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