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", more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. An estimated nine million combatants and seven million civilians died as a direct result of the war, while it is also considered a contributory factor in a number of genocides and the 1918 influenza epidemic, which caused between 50 to 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 the Second World War about twenty years later.
By 1914, the European powers 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, while many of the terms of both agreements were informal and contradicted by others; for example, Italy renewed the Triple Alliance in 1902 but secretly agreed with France to remain neutral if it was attacked by Germany. As the war widened, the Entente added Italy, Japan and eventually the United States to form the Allied Powers, while the Ottoman Empire and Bulgaria joined Germany and Austria to create the Central Powers.
Between 1908 and 1914, the Balkans had been destabilised by the combination of a weakened Ottoman Empire, the 1912–1913 Balkan Wars and competing Russian and Austro-Hungarian objectives. On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to a diplomatic crisis. On 23 July, Austria-Hungary issued an ultimatum to Serbia; interlocking alliances quickly drew in all the major European powers with their respective colonial empires and the conflict rapidly spread across the globe.
On 25 July, the Russian government issued orders for the 'period preparatory to war'; after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, partial mobilisation was approved of the military districts nearest to Austria, including Kiev, Kazan, Odessa and Moscow. 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, with Austria-Hungary following suit on 6th; France ordered full mobilisation in support of Russia on 2 August. French entry into the war stemmed from a combination of the desire to regain the provinces of Alsace-Lorraine ceded after the 1870–1871 Franco-Prussian War, concern at Germany's increasing power and military commitments agreed with Russia.
German strategy for a war on two fronts against France and Russia was to concentrate the bulk of their 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 entered 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, the Empire of Japan joined the Allied Powers, seizing the opportunity to expand its sphere of influence by capturing German possessions in China and the Pacific. On 24 August, Serbia won a major victory over the Austro-Hungarians at the Battle of Cer.
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. On the Eastern Front, two Russian armies entered East Prussia on 17 August, in compliance with their 1912 agreement with France to attack Germany within 15 days of mobilisation. The Germans were forced to divert troops from the West but successfully repulsed this invasion by victories at Tannenberg and the Masurian Lakes; however, the Russians occupied the Austro-Hungarian province of Eastern Galicia.
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. Romania joined the Allied Powers in 1916. 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 Russian government collapsed in March 1917 with the February Revolution, and the October Revolution followed by a further military defeat brought the Russians to terms with the Central Powers via the Treaty of Brest-Litovsk, which granted the Germans a significant victory. After the stunning German Spring Offensive along the Western Front in the spring of 1918, the Allied Powers rallied and drove back the Germans in the successful Hundred Days Offensive. On 4 November 1918, the Austro-Hungarian empire agreed to the Armistice of Villa Giusti, and Germany, which had its own trouble with revolutionaries, agreed to an armistice on 11 November 1918, ending the war in victory for the Allied Powers.
By the end of the war or soon after, the German Empire, Russian Empire, Austro-Hungarian Empire and Ottoman Empire ceased to exist. National borders were redrawn and Germany's colonies were parcelled out among the victors. During the Paris Peace Conference of 1919, the Big Four powers (Britain, France, the United States and Italy) imposed their terms in a series of treaties. The League of Nations was formed with the aim of preventing any repetition of such a conflict. This effort failed, however, as weakened successor states, renewed nationalism, economic depression, and feelings of humiliation (particularly in Germany) eventually contributed to the start of World War II.