What became the most immediate cause for the First World War?
Or
What was the Sarajevo Crisis?

On 28 June, 1914, the Archduke Francis Ferdinand, the Crown Prince of Austria and his wife were assassinated at Sarajevo, the Capital City of Bosnia. Though the assassin, Gavrilo Princep was an Austrian subject, the assassination had been planned in Serbia by a secret society of patriotic terrorists called 'Black Hand'.

Serbia was blamed for Archduke's assassination and Austria declared war on Serbia. Russia supported Serbia, Germany declared war on Russia and Great Britain declared war on Germany. Italy left the Triple Alliance and joined the war against Germany. Thus, the First World War began.