World War I (WWI or WW1), also known as the First World War, the Great War, or the War to End All Wars,
 was a
global war originating in
Europe that lasted from 28 July 1914 to 11 November 1918. More than 70 million
military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. Over nine million
combatants and seven million
died as a result of the war (including the victims of a
number of genocides), a casualty rate exacerbated by the belligerents'
technological and industrial sophistication, and the tactical stalemate caused by gruelling
trench warfare. It was
one of the deadliest conflicts in history and precipitated major political change, including
revolutions in many of the nations involved. Unresolved rivalries at the end of the conflict contributed to the start of the
Second World War twenty-one years later.
The war drew in all the world's economic
great powers, assembled in two opposing alliances: the
Allies (based on the
Triple Entente of the
Russian Empire, the
French Third Republic, and the
United Kingdom of Great Britain and Ireland) versus the
Central Powers of
Italy was a member of the
Triple Alliance alongside Germany and Austria-Hungary, it did not join the Central Powers, as Austria-Hungary had taken the offensive against the terms of the alliance.
 These alliances were reorganised and expanded as more nations entered the war: Italy,
Japan and the
United States joined the Allies, while the
Ottoman Empire and
Bulgaria joined the Central Powers.
The trigger for the war was the
assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria-Hungary, by
Gavrilo Princip in
Sarajevo on 28 June 1914. This set off a
diplomatic crisis when Austria-Hungary delivered an ultimatum to the
Kingdom of Serbia,
 and entangled international alliances formed over the previous decades were invoked. Within weeks the major powers were at war, and the conflict soon spread around the world.
Russia was the first to order a partial mobilization of its armies on 24–25 July, and when on 28 July Austria-Hungary declared war on Serbia, Russia declared general mobilization on 30 July.
 Germany presented an ultimatum to Russia to demobilise, and when this was refused, declared war on Russia on 1 August. Being outnumbered on the
Eastern Front, Russia urged its Triple Entente ally France to open up a second front in the west. Over forty years earlier in 1870, the
Franco-Prussian War had ended the
Second French Empire and France had ceded the provinces of
Alsace-Lorraine to a unified Germany. Bitterness over that defeat and the determination to retake Alsace-Lorraine made the acceptance of Russia's plea for help an easy choice, so France began full mobilisation on 1 August and, on 3 August, Germany declared war on France. The border between France and Germany was heavily fortified on both sides so, according to the
Schlieffen Plan, Germany then
invaded neutral Belgium and
Luxembourg before moving towards France from the north, leading the United Kingdom to declare war on Germany on 4 August due to their violation of Belgian neutrality. After the German march on Paris was halted in the
Battle of the Marne, what became known as the
Western Front settled into a
battle of attrition, with a
trench line that changed little until 1917. On the
Eastern Front, the Russian army led a successful campaign against the Austro-Hungarians, but the Germans stopped its
invasion of East Prussia in the battles of
Tannenberg and the
Masurian Lakes. In November 1914, the Ottoman Empire joined the Central Powers, opening fronts in the
Mesopotamia and the
Sinai. In 1915, Italy joined the Allies and Bulgaria joined the Central Powers;
Romania joined the Allies in 1916, as did the United States in 1917.
The Russian government
collapsed in March 1917, and
a revolution in November 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 a
stunning German offensive along the Western Front in the spring of 1918, the Allies rallied and drove back the Germans in a
series of successful offensives. On
4 November 1918, the Austro-Hungarian empire agreed to an armistice, and Germany, which had
its own trouble with revolutionaries, agreed to an armistice on 11 November 1918, ending the war in victory for the Allies.
By the end of the war or soon after, the German Empire, Russian Empire,
Austro-Hungarian Empire and the Ottoman Empire ceased to exist. National borders were redrawn, with 9 independent nations restored or created,
Germany's colonies were parceled out among the victors. During the
Paris Peace Conference of 1919, the
Big Four (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, and economic depression, renewed nationalism, weakened successor states, and feelings of humiliation (particularly in Germany) eventually contributed to the start of
World War II.