World War I (WWI) or the First World War, formerly called the Great War, was a major war
centred on Europe that began in the summer of 1914 and lasted until November 1918. It
involved all of the world's great powers,[4] assembled in two opposing alliances:
the Allies (centred around the Triple Entente) and the Central Powers.[5] More than 70 million
military personnel, including 60 million Europeans, were mobilised in one of the largest wars
in history.[6][7] More than 9 million combatants were killed, largely because of great
technological advances in firepower without corresponding advances in mobility. It was the
seconddeadliest conflict in Western history.[8]
The assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria, the heir to the
throne of Austria-Hungary, was the proximate trigger of the war. Long-term causes, such
as imperialistic foreign policies of the great powers of Europe, such as theGerman Empire,
the Austro-Hungarian Empire, the Ottoman Empire, the Russian Empire, the British
Empire, France, and Italy, played a major role. Ferdinand's assassination by a Yugoslav
nationalist resulted in a Habsburg ultimatum against the Kingdom of Serbia.[9][10] Several
alliances formed over the past decades were invoked, so within weeks the major powers were
at war; via their colonies, the conflict soon spread around the world.
On 28 July, the conflict opened with the Austro-Hungarian invasion of Serbia,[11][12] followed by
the German invasion of Belgium,Luxembourg and France; and a Russian attack against
Germany. After the German march on Paris was brought to a halt, theWestern Front settled
into a static battle of attrition with a trench line that changed little until 1917. In the East, the
Russian army successfully fought against the Austro-Hungarian forces but was forced back
by the German army. Additional fronts opened after the Ottoman Empire joined the war in
1914, Italy and Bulgaria in 1915 and Romania in 1916. The Russian Empire collapsed in
1917, and Russia left the war after the October Revolution later that year. After a 1918
German offensive along the western front, United States forces entered the trenches and the
Allies drove back the German armies in a series of successful offensives. Germany agreed to
a cease-fire on 11 November 1918, later known as Armistice Day.
By the war's end, four major imperial powers—the German, Russian, Austro-Hungarian and
Ottoman Empires—had been militarily and politically defeated. The latter two ceased to exist.
[13]
The revolutionised Soviet Union emerged from the Russian Empire, while the map of
central Europe was completely redrawn into several smaller states.[14] The League of
Nations was formed in the hope of preventing another such conflict. The European
nationalism spawned by the war and the breakup of empires, and the repercussions of
Germany's defeat and the Treaty of Versailles led to the beginning of World War II in 1939.