The Serbian Campaign (World War I) was fought from August 1914, when Austria-Hungary invaded Serbia at the outset of First World War, until the end of the war in 1918. The front ranged from the Danube to southern Macedonia and back north again, involving forces from almost all combatants of the war.
The Serbian Army was decimated towards the end of the war, falling from about 420,000 at its peak to about 100,000 at the moment of liberation. The Kingdom of Serbia had lost 725,000 inhabitants during the war (both army and civilian losses), which represented over 16% of its overall population. (According to the Yugoslav government in 1924: Serbia lost 365,164 soldiers, or 26 percent, of all mobilized people [for ex. France 16.8; Germany 15.4; Russia 11.5; Italy 10.3 per cent]).
World War I was, in at least one sense, started when Chief of Serbian Military Intelligence Dragutin Dimitriević ordered the assassination of Archduke Franz Ferdinand heir to the throne of Austria-Hungary. Dimitriević's right hand man, Major Tankosić armed and trained three assassins and arranged for their clandestine transport across the border and to Sarajevo. In Sarajevo, Danilo Ilić had been alerted by Belgrade to arrange additional local assassins and he recruited an additional three arming them with the weapons brought from Belgrade. The attack killed the archduke and his wife and wounded 20. The assassins were arrested and gave up the names of the many Serbian military officers who had helped them on their way. Austria-Hungary requested Serbia open an investigation parallel to their own on Serbian soil, but Serbia flatly rejected the request.