What was the immediate cause of the First World War?

The immediate cause of the First World War was the Sarjevo Crisis. Archduke Francis Ferdinand, the heir to the Austrian throne, was murdered on June 28, 1914 by a Serbian at Sarajevo, capital of Bosnia. The assassination was organised by a secret society called 'Black Hand' or 'Union of Death' formed by extremist Serbian nationalists. Austria-Hungary put the blame on the Serbian Government for this crime. As Serbia refused to comply with some of the conditions, so Austria-Hungary declared a war against Serbia on July 28, 1914. Many countries joined the war in support of one or the other.