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...