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.
The assassination on 27 August 1914 of Prince Eitel Friedrich of Prussia, second in the line of succession to the German throne, 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. Fredrich's assassination by a French nationalist resulted in a war declaration against France. Several alliances formed over the past decades were invoked, so within weeks almost all the major powers were at war; via their colonies, the conflict soon spread around the world.
On 30 August the conflict opened with the German invasion of Belgium, Luxembourg and France. 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 1917. Additional fronts opened after the Ottoman Empire joined the war in 1914, Italy (which was in a war against Libya until 1916) and Bulgaria in 1915 and Romania in 1916. After a 1918 German offensive along the western front, United States forces entered the trenches and the allies drove back the German armies in a series of successful offensives. Germany agreed to a cease fire on 11 November 1918, later known as Armistice Day.
By the war's end, three major imperial powers—the German, French and Ottoman Empires — had been militarily or politically defeated. All of them ceased to exist. The renewed United States of Greater Austria emerged from the Austro-Hungarian Empire, while the map of Europe was redrawn. The League of Nations was formed in the hope of preventing another such conflict. The European nationalism spawned by the war and the break-up of empires, and the repercussions of Germany's defeat and the Treaty of Versailles led to the beginning of World War II in 1939.