World War I, also known as the Great War, was fought from 28 July 1914 to 11 November 1918. It pitted the Allied Powers—principally France, the Russian Empire, the British Empire, Italy (from 1915), and the United States (from 1917)—against the Central Powers: Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria.
The immediate trigger was the assassination of Archduke Franz Ferdinand of Austria in Sarajevo on 28 June 1914 by Gavrilo Princip. Austria-Hungary's ultimatum to Serbia activated a web of alliances, mobilization schedules, and imperial rivalries that turned a Balkan crisis into a continental and then global war. Underlying causes commonly cited by historians include militarism, entangling alliances, imperial competition, and nationalism.
The war introduced industrialized mass killing: trench warfare on the Western Front, machine guns, poison gas (first large-scale use by Germany at Ypres in April 1915), tanks, submarines, and aerial bombardment. Major engagements included the Marne (1914), Gallipoli (1915–16), Verdun and the Somme (1916), and Passchendaele (1917). Estimated military deaths exceed 9 million, with civilian deaths and the 1918 influenza pandemic pushing total mortality far higher.
The war ended with the armistice of 11 November 1918 and was formally concluded by a series of treaties, most famously the Treaty of Versailles (28 June 1919) with Germany, alongside Saint-Germain-en-Laye (Austria), Trianon (Hungary), Neuilly (Bulgaria), and Sèvres/Lausanne (Ottoman Empire/Turkey).
Consequences central to IR study include:
- Collapse of the German, Austro-Hungarian, Russian, and Ottoman empires.
- Creation of the League of Nations (1920), the first permanent intergovernmental organization for collective security.
- Redrawing of European and Middle Eastern borders, including the mandate system under League Covenant Article 22.
- Reparations and territorial clauses that conditioned the interwar period and contributed to the conditions for World War II.
It remains a foundational case study in alliance dynamics, deterrence failure, and post-war institutional design.
Example
In 1917, the United States entered World War I on the Allied side following Germany's resumption of unrestricted submarine warfare and the disclosure of the Zimmermann Telegram.
Frequently asked questions
It began on 28 July 1914 with Austria-Hungary's declaration of war on Serbia and ended with the armistice on 11 November 1918; formal peace treaties followed in 1919–1923.
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