World War I or the First World War, often abbreviated as WWI or WW1, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously known as the Great War or “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, and also one of the deadliest conflicts in history, with an estimated 8.5 million combatant deaths and 13 million civilian deaths as a direct result of the war. Resulting genocides and the related 1918 Spanish flu pandemic caused many millions of deaths worldwide.

On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist and member of the Serbian Black Hand military society, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis. In response, Austria-Hungary issued an ultimatum to Serbia on 23 July. 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 July 1914, the great powers of Europe were divided into two coalitions: the Triple Entente, consisting of France, Russia, and Britain; and the preestablished Triple Alliance of Germany, Austria-Hungary, and Italy. The Triple Alliance was only defensive in nature, allowing Italy to stay out of the war until 26 April 1915, when it joined the Allied Powers after its relations with Austria-Hungary deteriorated. Russia felt it necessary to back Serbia, and approved partial mobilisation after Austria-Hungary shelled the Serbian capital of Belgrade, which was a few kilometres from the border, on 28 July 1914. Full Russian mobilisation was announced on the evening of 30 July; the following day, Austria-Hungary and Germany did the same, while Germany demanded Russia demobilise within twelve hours. When Russia failed to comply, Germany declared war on Russia on 1 August 1914 in support of Austria-Hungary, the latter following suit on 6 August 1914. France ordered full mobilisation in support of Russia on 2 August 1914. In the end, World War I would see the continent of Europe split into two major opposing alliances; the Allied Powers, primarily composed of the United Kingdom of Great Britain and Ireland, the United States, France, the Russian Empire, Italy, Japan, Portugal, Greece, Serbia and Montenegro; and the Central Powers, primarily composed of the German Empire, the Austro-Hungarian Empire, the Ottoman Empire and Bulgaria.

Germany’s strategy for a war on two fronts against France and Russia was to rapidly concentrate the bulk of its army in the West to defeat France within 6 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 on 3 August and declared war on France the same day; the Belgian government invoked the 1839 Treaty of London and, in compliance with its obligations under this treaty, Britain declared war on Germany on 4 August. On 12 August, Britain and France also declared war on Austria-Hungary; on 23 August, Japan sided with Britain, seizing German possessions in China and the Pacific. In November 1914, the Ottoman Empire entered the war on the side of Austria-Hungary and Germany, opening fronts in the Caucasus, Mesopotamia, and the Sinai Peninsula. The war was fought in (and drew upon) each power’s colonial empire also, spreading the conflict to Africa and across the globe.

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 war of attrition, marked by a long series of trench lines that changed little until 1917 (the Eastern Front, by contrast, was marked by much greater exchanges of territory). In 1915, Italy joined the Allied Powers and opened a front in the Alps. Bulgaria joined the Central Powers in 1915 and Greece joined the Allies in 1917, expanding the war in the Balkans. The United States initially remained neutral, though even while neutral it became an important supplier of war materiel to the Allies. Eventually, after the sinking of American merchant ships by German submarines, the declaration by Germany that its navy would resume unrestricted attacks on neutral shipping, and the revelation that Germany was trying to incite Mexico to initiate war against the United States, the U.S. declared war on Germany on 6 April 1917. Trained American forces did not begin arriving at the front in large numbers until mid-1918, but the American Expeditionary Force ultimately reached some two million troops.

Increasing war-weariness in Russia led to the 1917 February Revolution, with the Tsar replaced by a Provisional Government, who remained committed to the war. However, widespread desire for peace resulted in the October Revolution, in which the Bolsheviks seized power; in March 1918, they signed the Treaty of Brest-Litovsk, taking Russia out of the war. Large numbers of German combat troops were transferred to the Western Front, where they took part in the March 1918 German Spring Offensive. Despite initial success, they were soon halted by ferocious defence and heavy casualties; with American reinforcements running at 10,000 men per day, German manpower reserves were exhausted. In August, the Allies launched the Hundred Days Offensive and although the German army continued to fight hard, it could no longer halt their advance. The Central Powers began to collapse; Bulgaria was the first to sign an Armistice on 29 September, followed by the Ottomans on 31 October, then Austria-Hungary on 3 November. Isolated, facing revolution at home and an army on the verge of mutiny, Kaiser Wilhelm abdicated on 9 November and the new German government signed an armistice on 11 November 1918, bringing the fighting to a close.

World War I was a significant turning point in the political, cultural, economic, and social climate of the world. The war and its immediate aftermath sparked numerous revolutions and uprisings. The Big Four (Britain, France, the United States, and Italy) imposed their terms on the defeated powers in a series of treaties agreed at the 1919 Paris Peace Conference, the most well known being the Treaty of Versailles with Germany. Ultimately, as a result of the war, the Austro-Hungarian, German, Ottoman, and Russian Empires ceased to exist, and numerous new states were created from their remains. However, despite the conclusive Allied victory (and the creation of the League of Nations during the peace conference, intended to prevent future wars), a second world war followed just over twenty years later. (Wikipedia)