The spark comes on 28th June, in the city of Sarajevo. Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, is assassinated by a 19 year-old Slav nationalist named Gavrilo Princip. Austro-Hungary accuses its Balkan rival Serbia of having aided the assassin, and sends an ultimatum, demanding humiliating concessions. Serbia rejects the ultimatum, and Austro-Hungary declares war. Within hours Austrian forces are shelling Belgrade.