World War I, also known as the First World War, the Great War and the War To End All Wars, was a global military conflict 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 immediate cause of the war was the June 28, 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 Black Hand. The retaliation by Austria-Hungary against 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. Ashworth, Tony. Trench warfare 1914–1918, pp3–4. 2000: Macmillan Press, London. These fortifications stretched 475 miles (more than 600 kilometres) and precipitated a style of fighting known as trench warfare. On the Eastern Front, the vast eastern plains and limited rail network prevented a trench warfare stalemate, though the scale of the conflict was just as large as on the Western Front. The Middle Eastern Front and the Italian Front also saw heavy fighting, while hostilities also occurred at sea, and for the first time, in the air.

The war was ended by several treaties, most notably the Treaty of Versailles, signed on June 28, 1919, though the Allied powers had an armistice with Germany in place since November 11, 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, 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 marked the end of the world order which had existed after the Napoleonic Wars, and was an important factor in the outbreak of World War II.