World War I : Military conflict, from 1914 to 1918, that began as a local European war between Austria-Hungary and Serbia on July 28, 1914; was transformed into a general European struggle by declaration of war against Russia on August 1, 1914; and eventually became a global war involving 32 nations. Twenty-eight of these nations, known as the Allies and the Associated Powers, and including Great Britain, France, Russia, Italy, and the United States, opposed the coalition known as the Central Powers, consisting of Germany, Austria-Hungary, Turkey, and Bulgaria. The immediate cause of the war between Austria-Hungary and Serbia was the assassination on June 28, 1914, at Sarajevo in Bosnia (then part of the Austro-Hungarian Empire; now in Bosnia and Herzegovina), of Archduke Francis Ferdinand, heir-presumptive to the Austrian and Hungarian thrones, by Gavrilo Princip, a Serb nationalist. The fundamental causes of the conflict, however, were rooted deeply in the European history of the previous century, particularly in the political and economic policies that prevailed on the Continent after 1871, the year that marked the emergence of Germany as a great world power.

Causes of the War : The underlying causes of World War I were the spirit of intense nationalism that permeated Europe throughout the 19th and into the 20th century, the political and economic rivalry among the nations, and the establishment and maintenance in Europe after 1871 of large armaments and of two hostile military alliances.

Summary of the War : World War I began on July 28, 1914, with the declaration of war by Austria-Hungary on Serbia, and hostilities between the Allied and Central Powers continued until the signing of the armistice on November 11, 1918, a period of 4 years, 3 months, and 14 days. The aggregate direct war costs of all the belligerents amounted to about $186 billion. Casualties in the land forces amounted to more than 37 million; in addition, close to 10 million deaths among the civilian populations were caused indirectly by the war. Despite worldwide hopes that the settlements arrived at after the war would restore world peace on a permanent basis.