World War I was a global war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. Long-term causes of the war included the imperialistic foreign policies of the great powers of Europe. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies and the Central Powers. These alliances both reorganised, and expanded as more nations entered the war. More than 9 million combatants were killed, largely because of enormous increases in lethality of weapons, thanks to new technology, without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved. By the end of the war, four major imperial powers ceased to exist. The League of Nations was formed in the hope of preventing another such conflict.