World War I, also known as the First World War and the Great War, was a global military conflict which took place primarily in Europe from 1914–1918. Over 40 million casualties resulted, including approximately 20 million military and civilian deaths. Over 60 million European soldiers were mobilized from 1914–1918. 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.