World War I began on July 28, 1914 and lasted until November 11, 1918. The conflict involved the great powers around the world, organized into two opposing alliances: the Allies, based in the Triple Entente between Britain, France and the Russian Empire and the Central Powers, originally Triple Alliance between German Empire, Austria-Hungary and Italy, but as Austria-Hungary had taken the offensive against the agreement, Italy has not gone to war. These alliances reorganized and Italy fought for the Allies expanding into more nations entered the war. Ultimately, 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, largely because of technological advances that led to a huge increase in the lethality of weapons, but without corresponding improvements in protection and mobility. It was the sixth deadliest conflict in human history and that later paved the way for various political changes such as revolutions in many of the nations involved.
Among the causes of war is included foreign imperialist 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, the French Third Republic and Italy. On June 28, 1914, the assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria-Hungary, by a Yugoslav nationalist in Sarajevo, Bosnia, was the immediate trigger of the war. 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 planet.
On 28 July, the conflict opened with the Austro-Hungarian invasion of Serbia, followed by the German invasion of Belgium, Luxembourg and France, and a Russian attack against Germany. After the German march on Paris have led to an impasse, the Western Front settled into a static battle of attrition with a trench line that changed little until 1917. On the Eastern Front, the Russian army successfully fought against the Austro-Hungarian forces but was forced to retreat from East Prussia and Poland by the German army. Additional battle fronts opened after the Ottoman Empire joined the war in 1914, Italy and Bulgaria in 1915 and Romania in 1916. The Russian Empire collapsed in March 1917, and Russia left the war after the October Revolution later that year. After a 1918 German offensive along the Western Front, the Allies forced the retreat of the German armies in a series of successful offensives and U.S. forces began to enter the trenches. Germany, which had its own trouble with revolutionaries at this point, agreed to a ceasefire on November 11, 1918, episode later known as Armistice Day. The war ended with the victory of the Allies.
Events in local conflicts were as tumultuous as the major battlefronts, and participants attempted to mobilize their manpower and financial resources to fight a total war. By the war's end, four major imperial powers - the German, Russian, Austro-Hungarian and Ottoman empires - no longer exist. The successors of the first two states have lost a lot of its territory, while the latter two were dismantled entirely. The map of central Europe was redrawn into several smaller countries. The League of Nations, the precursor organization of the United Nations was formed in the hope of avoiding another conflict of this magnitude. There is consensus that European nationalism provoked by the war, the breakup of empires, the repercussions of Germany's defeat and problems with the Treaty of Versailles were factors that contributed to the outbreak of the World War II.