World War I, also called the World War or the Great War, was a world war that began in Europe on July 28, 1914 and lasted until November 11, 1918. November 11 remained known as Armistice Day. All the major powers of the world were involved in this war and were composed of two conflicting alliances: the Allies (centered around the Triple Entente of the United Kingdom, France and Russia) and the Centrals (originally centered around the Triple Alliance of Germany, Austria-Hungary and Italy). These alliances reorganized (Italy defected to the Allies in 1915) and expanded as more countries joined the war (Romania joined the Allies