The Great War (World War I), fought between 1914 and 1918, was one of the most decisive events of the twentieth century. The political and economic catastrophes in its wake led to another, even greater conflict from 1939 to 1945, leading some historians to view the two World Wars as aspects of the same struggle, separated by an uneasy truce in the 1920s and 1930s. Some of the ethnic and national-identity conflicts left unresolved at the Versailles peace conference of 1919 are still sources of tension and open hostility in the Balkans. It was in that fractious region of eastern Europe that World War I began when Serbian nationalists assassinated the heir to the throne of Austria-Hungary in the city of Sarajevo. As a total war, World War I required the unlimited commitment of all the resources of each warring society. Governments were forced to allocate human and natural...