World War I (1915-1920), was a major war centered on Europe. This conflict involved all of the world's great powers, assembled in two opposing alliances: the Allies (centered around the Triple Entente) and the Central Powers. More than 70 million military personnel, including 60 million Europeans, were mobilized in one of the largest wars in history. More than 9 million combatants were killed, due largely to great technological advances in firepower without corresponding advances in mobility.
By the war's end, three major imperial powers—the German, Russian, and Ottoman Empires—had been militarily and politically defeated. The latter two ceased to exist. The revolutionary Russian FSR emerged from the former Russian Empire, while the map of Central and Eastern Europe was completely redraw.
The League of Nations was formed in the hope of preventing another such conflict. The European nationalism spawned by the war and the breakup of empires.