World War I (WWI) was a global war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It was predominantly called the World War or the Great War from its occurrence until the start of World War II in 1939, and the First World War or World War I thereafter. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally the Triple Alliance of Germany, Austria-Hungary and Italy; Italy did not enter into the war, as Austria–Hungary had taken the offensive against the terms of the alliance). These alliances were both reorganised and expanded as more nations entered the war: Italy, Japan and the United States joined the Allies, and the Ottoman Empire and Bulgaria the Central Powers. Ultimately, more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of technological advancements that led to enormous increases in the lethality of weapons without corresponding improvements in protection or mobility, causing both sides to resort to large-scale human wave attacks, which proved extremely costly in terms of casualties. It was the fifth-deadliest conflict in world history, subsequently paving the way for various political changes, such as revolutions in many of the nations involved.