World War I (also known as the First World War and the Great War) was a global military conflict that embroiled most of the world's great powers, assembled in two opposing alliances: the Entente and the Central Powers. The immediate cause of the war was the June 28, 1914 assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by Gavrilo Princip, a Bosnian Serb citizen of Austria–Hungary and member of the Black Hand. The retaliation by Austria–Hungary against Serbia activated a series of alliances that set off a chain reaction of war declarations. Within a month, much of Europe was in a state of open warfare, resulting in the mobilization of more than 65 million European soldiers, and more than 40 million casualties—including approximately 20 million deaths by the en