World War I (also known as the First World War and the Great War) was a global military conflict that involved most of the world's great powers. The war was assembled in two opposing groups: the Entente and the Central Powers. The immediate cause of the war was the June 28, 1914 assassination of Archduke Franz Ferdinand. He was heir to the Austro-Hungarian throne. He was killed by Gavrilo Princip who was a Bosnian Serb citizen of Austria–Hungary. He was also a 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. This resulted in the mobilization of more than 65 million European soldiers, and more than 40 million casualties—including approximately 20 million deaths by the end of the war.