 On 28 June 1914 , Bosnian Serb student Gavrilo Princip assassinated Archduke Franz Ferdinand of Austria in Sarajevo . The assassination precipitated the July Crisis , which led Austria @-@ Hungary to issue an ultimatum to Serbia on 23 July on suspicion that the assassination had been planned in Belgrade . The Austro @-@ Hungarian government made the ultimatum intentionally unacceptable to Serbia , and it was indeed rejected . The Austro @-@ Hungarians declared war on Serbia on 28 July and that same day the Serbs destroyed all bridges on the Sava and Danube rivers in order to prevent the Austro @-@ Hungarians from using them during any future invasion . Belgrade was shelled the following day , marking the beginning of World War I. 
