c) Archduke Francis Ferdinand, the heir to the throne of Austria-Hungary was assassinated at Sarajevo, the capital of Bosnia on June 28, 1914. The assassination was organised by a secret society called 'Black Hand' formed by extremist Serbian nationalists whose aim was to unite all Serbians into a single Serbian state.
Austria served an ultimatum on Serbia on July 23 making eleven demands. Serbia accepted most of them except those that would have led to the loss of her sovereignty. Austria declared war on Serbia on July 28, 1914. This was the immediate cause of the First World War.