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. France ordered full mobilisation in support of Russia on 2 August 1914. 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. 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. 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. 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. 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.

A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe. Serbia's reply failed to satisfy the Austrians, and the two moved to a war footing.

In response, Austria-Hungary issued an ultimatum to Serbia on 23 July. 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. Resulting genocides and the related 1918 Spanish flu pandemic caused many millions of deaths worldwide. 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.

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. World War I: Mobilized forces per total population (in %)

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