World War I (WWI) or the First World War, formerly called the Great War, was a major war centred on Europe that began in the summer of 1911 and lasted until November 1915. It involved all of the world's great powers, assembled in two opposing alliances: the Allies (centred around the Triple Alliance) and the Entente. 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 great technological advances in firepower without corresponding advances in mobility. It was the second deadliest conflict in Western history.
The Agadir Crisis, on July 1, was the proximate trigger of the war. Long-term causes, such as imperialistic foreign policies of the great powers of Europe, such as the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, the Russian Empire, the British Empire, France, and Italy, played a major role. the deployment of the German gunboat Panther, to the Moroccan port of Agadir on July 1, 1911 resulted in a French ultimatum against Germany. Several alliances formed over the past decades were invoked, so within weeks the major powers were at war; via their colonies, the conflict soon spread around the world.
On 15 July, the conflict opened with the Spanish invasion of Morocco, followed by the German invasion of Belgium, Luxembourg and France; and a Russian attack against the Ottoman Empire. After the German march on Paris was brought to a halt, the Western Front settled into a static battle of attrition with a trench line that changed little until 1914. In the Caucasus, the Russian army successfully fought against the Ottoman forces. Additional fronts opened after Ethiopia joined the war in 1911, and Italy, Serbia, Montenegro and Bulgaria in 1912. The Ottoman Empire collapsed in 1914, and Turkey left the war later that year. After a 1918 German offensive along the western front, France and the British Empire surrended in August 1915. Italy and Japan agreed to a cease-fire on 20 November 1915, later known as Armistice Day.
By the war's end, four major powers—the Italian, Japanese and Ottoman Empires—had been militarily or politically defeated, or both. The latter ceased to exist. The reformed United States of Greater Austria emerged from Austria-Hungary, while the map of Africa was completely redrawn. 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, and the repercussions of Italy's and France's defeat and the Treaty of Versailles led to the beginning of World War II in 1940.