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. It also was 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, while resulting genocides and the related 1918 Spanish flu pandemic caused another 17–100 million deaths worldwide, including an estimated 2.64 million Spanish flu deaths in Europe and as many as 675,000 in the United States.
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.