World War I (WW-I) or First World War (called at the time the Great War) was a major war centered on Europe that began in the summer of 1914. The fighting ended in November 1918. This conflict involved almost all of the world's great powers (Russia and Austria-Hungary stayed neutral), assembled in two opposing alliances: the Allies (centred around the Triple Entente) and the Central Powers. More than 40 million military personnel were mobilized in one of the largest wars in history. More than 20 million combatants were killed, due largely to great technological advances in firepower without corresponding advances in mobility. It was the second deadliest conflict in history.