World War I (abbreviated WWI; also known as the First World War, the Great War, and the War to End All Wars) was a global war which took place primarily in Europe from 1914 to 1918.  Over 40 million casualties resulted, including approximately 20 million military and civilian deaths.  Over 60 million European soldiers were mobilized from 1914 to 1918.  The act which is considered to have triggered the succession of events which led to war was the 28 June 1914 assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by Gavrilo Princip, a Bosnian Serb citizen of Austria-Hungary and member of the Young Bosnia.
The retaliation by Austria-Hungary against the Kingdom of Serbia activated a series of alliances that set off a chain reaction of war declarations. Within a month, much of Europe was in a state of open warfare. The war was propagated by two major alliances. The Entente Powers initially consisted of France, the United Kingdom, Russia, and their associated empires and dependencies. Numerous other states joined these allies, most notably Italy in April 1915, and the United States in April 1917.
The Central Powers, so named because of their central location on the European continent, initially consisted of Germany and Austria-Hungary and their associated empires. The Ottoman Empire joined the Central Powers in October 1914, followed a year later by Bulgaria. By the conclusion of the war, only The Netherlands, Switzerland, Spain and the Scandinavian nations remained officially neutral among the European countries, though many of those provided financial and material support to one side or the other.
The fighting of the war mostly took place along several fronts that broadly encircled the European continent. The Western Front was marked by a system of trenches, breastworks, and fortifications separated by an area known as no man’s land.  These fortifications stretched 475 miles (more than 600 kilometres) and precipitated a style of fighting known as trench warfare. On the Eastern Front, the vastness of the eastern plains and the limited railroad network prevented the stalemate of the Western Front, though the scale of the conflict was just as large.
There was heavy fighting on the Balkan Front, the Middle Eastern Front and the Italian Front; there were also hostilities at sea and in the air. The war was ended by several treaties, most notably the Treaty of Versailles, signed on 28 June 1919, though the Allied powers had an armistice with Germany in place since 11 November 1918. One of the most striking results of the war was a large redrawing of the map of Europe. All of the Central Powers lost territory, and many new nations were created.
The German Empire lost its colonial possessions and was saddled with accepting blame for the war, as well as paying punitive reparations for it. The Austro-Hungarian and Ottoman empires were completely dissolved. Austria-Hungary was carved up into several successor states including Austria, Hungary, Czechoslovakia, and Yugoslavia. The Ottoman Empire disintegrated, and much of its non-Anatolian territory was awarded as protectorates of various Allied powers, while the remaining Turkish core was reorganized as the Republic of Turkey.
The Russian Empire, which had withdrawn from the war in 1917 after the October Revolution, lost much of its western frontier as the newly independent nations of Estonia, Finland, Latvia, Lithuania, and Poland were carved from it. After the war, the League of Nations was created as an international organization designed to avoid future wars by giving nations a means of solving their differences diplomatically. World War I ended the world order which had existed since the end of the Napoleonic Wars, and was an important factor in the outbreak of World War II.