World War I, military conflict, from August 1914 to November 1918, that involved many of the countries of Europe as well the United States and other nations throughout the world. World War I was one of the most violent and destructive wars in European history. Of the 65 million men who were mobilized, more than 10 million were killed and more than 20 million wounded. The term World War I did not come into general use until a second worldwide conflict broke out in 1939 Before that year, the war was known as the Great War or the World War.
World War I was the first total war. Once the war began, the countries involved mobilized their entire populations and economic resources to achieve victory on the battlefield. The term home front, which was widely employed for the first time during World War I, perfectly symbolized this new concept of a war in which the civilian population behind the lines was directly and critically involved in the war effort.
The war began as a clash between two coalitions of European countries. The first coalition, known as the Allied Powers, included the United Kingdom, France, Belgium, Serbia, Montenegro, and the Russian Empire .The Central Powers, which opposed them, consisted of the empires of Germany and Austria-Hungary. Japan joined the Allied Powers in 1914. The Ottoman Empire joined the Central Powers in 1914, as did Bulgaria in 1915. The same year, Italy entered the war on the Allied side. Although the United States initially remained neutral, it joined the Allies in 1917. The conflict eventually involved 32 countries, 28 of which supported the Allies. Some of these nations, however, did not participate in the actual fighting.
The immediate cause of the war was the assassination of Archduke Francis Ferdinand,