How did the assassination of Archduke Ferdinand start the chain reaction that caused World War I? Not a multiple choice, I just need info on the political alliances and who the assassin worked for / country of origin

The direct cause of WWI was the assassination of Archduke Franz Ferdinand at Sarajevo on 28 June 1914. However historians feel that a number of factors contributed to the rivalry between the Great powers that allowed war on such a wide-scale to break out. I'm not sure who the assassin was.