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Archduke Franz Ferdinand heir to the throne of the Austro-Hungarian Empire, and his wife are assassinated in Sarajevo.
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The simplest answer is that the immediate cause was the assassination of Franz Ferdinand, the archduke of Austria-Hungary.
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Austria-Hungary declares war on Serbia
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As an ally of Serbia, Russia announces full mobilization of her armed forces.
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Germany mobilizes her armed forces and declares war on Russia.
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Germany declares war on France.
