World War I (often abbreviated as WWI or WW1), also known as the First World War or the Great War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously described as "the war to end all wars", it led to the mobilisation of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history. An estimated nine million combatants and seven million civilians died as a direct result of the war, and it also contributed to later genocides and the 1918 influenza pandemic, which caused between 50 and 100 million deaths worldwide. Military losses were exacerbated by new technological and industrial developments and the tactical stalemate caused by gruelling trench warfare. It was one of the deadliest conflicts in history and precipitated major political changes, including the Revolutions of 1917–1923, in many of the nations involved. Unresolved rivalries at the end of the conflict contributed to the start of World War II about twenty years later.
On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis. In response, on 23 July Austria-Hungary issued an ultimatum to Serbia. Serbia's reply failed to satisfy the Austrians, and the two moved to a war footing.
A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe. By 1914, the great powers of Europe were divided into two coalitions: the Triple Entente—consisting of France, Russia and Britain—and the Triple Alliance of Germany, Austria-Hungary and Italy (the Triple Alliance was primarily defensive in nature, allowing Italy to stay out of the war in 1914). Russia felt it necessary to back Serbia, on 25 July issuing orders for the 'period preparatory to war', and after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, partial mobilisation was approved of the military districts nearest to Austria. General Russian mobilisation was announced on the evening of 30 July; on the 31st, Austria-Hungary and Germany did the same, while Germany demanded Russia demobilise within 12 hours. When Russia failed to comply, Germany declared war on 1 August in support of Austria-Hungary, with Austria-Hungary following suit on 6th; France ordered full mobilisation in support of Russia on 2 August.
German strategy for a war on two fronts against France and Russia was to concentrate the bulk of its army in the West to defeat France within four weeks, then shift forces to the East before Russia could fully mobilise; this was later known as the Schlieffen Plan. On 2 August, Germany demanded free passage through Belgium, an essential element in achieving a quick victory over France. When this was refused, German forces invaded Belgium early on the morning of 3 August and declared war with France the same day; the Belgian government invoked the 1839 Treaty of London and in compliance with its obligations under this, Britain declared war on Germany on 4 August. On 12 August, Britain and France also declared war on Austria-Hungary; on the 23rd, Japan sided with the Entente, seizing the opportunity to expand its sphere of influence by capturing German possessions in China and the Pacific. The war was fought in and drew upon each powers' colonial empires as well, spreading the conflict across the globe. The Entente and its allies would eventually become known as the Allied Powers, while the grouping of Austria-Hungary and Germany would become known as the Central Powers.
The German advance into France was halted at the Battle of the Marne and by the end of 1914, the Western Front settled into a battle of attrition, marked by a long series of trench lines that changed little until 1917. The Eastern Front was marked by much greater exchanges of territory, but though Serbia was defeated in 1915, and Romania joined the Allied Powers in 1916 only to be defeated in 1917, none of the great powers were knocked out of the war until 1918. In November 1914, the Ottoman Empire joined the Central Powers, opening fronts in the Caucasus, Mesopotamia and the Sinai Peninsula. In 1915, Italy joined the Allied Powers and Bulgaria joined the Central Powers. After the sinking of seven US merchant ships by German submarines, and the revelation that the Germans were trying to incite Mexico to make war on the United States, the US declared war on Germany on 6 April 1917.
The 1917 February Revolution in Russia replaced the Tsarist autocracy with the Provisional Government, but continuing discontent at the cost of the war led to the October Revolution, the creation of the Soviet Socialist Republic, and the signing of the Treaty of Brest-Litovsk by the new government, ending Russia's involvement in the war. This allowed the transfer of large numbers of German troops from the East to the Western Front, resulting in the German March 1918 Offensive. The offensive was initially successful, but the Allied Powers rallied and drove the Germans back in the Hundred Days Offensive. Bulgaria was the first Central Power to sign an armistice—the Armistice of Salonica on 29 September 1918. On 30 October, the Ottoman Empire capitulated, signing the Armistice of Mudros. On 4 November, the Austro-Hungarian empire agreed to the Armistice of Villa Giusti. With its allies defeated, revolution at home, and the military no longer willing to fight, Kaiser Wilhelm abdicated on 9 November and Germany also signed an armistice on 11 November 1918.
World War I was a significant turning point in the political, cultural, economic, and social climate of the world. As a result of the war, the Russian Empire, the German Empire, Austria-Hungary and the Ottoman Empire ceased to exist. Revolutions and uprisings in the aftermath of the war became widespread, being mainly socialist or anti-colonial in nature. The Big Four (Britain, France, the United States and Italy) imposed their terms in a series of treaties agreed at the 1919 Paris Peace Conference. The formation of the League of Nations was intended to prevent another world war, but for various reasons failed to do so. The rise of the Nazi Party and their central role in World War II led to a focus on how the Treaty of Versailles affected Germany, but the peace treaties in addition to various secret agreements during the war also transformed borders throughout Europe, Asia and the Middle East, with repercussions that still echo to this day.