The Serbian War was the invasion of Serbia by Austria-Hungary. The Invasion of Serbia began on August 12, 1914. The Austro-Hungarian armies crossed the border into Serbia over the River Drina. Though Montenegro didn't declare war on Austria-Hungary, seeing the country as too large a threat to deal with on their own. This felt like a Great Betrayal to the Serbians at the time, the Balkan Betrayal. There were many reasons for the invasion of Serbia by Austria-Hungary. Though the main reason was due to the assassination of the Archduke Franz Ferdinand by a member of the Black Hand gang.