Gavrilo Princip was a Serbian terrorist who assassinated Archduke Franz Ferdinand of Austria-Hungary, the heir to the Austrian throne, in Sarajevo, Bosnia on 28 June 1914.
Princip killed the Archduke in protest of Austria's claims on Sarajevo and the rest of Bosnia, territories the Serbs believed were rightfully theirs. (PROSE: Human Nature)
Ferdinand's death led directly to the First World War, "through nations having treaties with nations", according to the Tenth Doctor. (TV: The Family of Blood)
Unbeknownst to the general populace, however, the start of the war was influenced by the passage of the Nemesis statue around Earth in November 1913. (TV: Silver Nemesis) Princip was merely the trigger.
The Eleventh Doctor began to suspect that Princip had been conditioned by the Daleks as part of their Dalek Project to kill Archduke Ferdinand and thus spark the war, although he never found any conclusive evidence to confirm this theory. (COMIC: The Dalek Project)