Large group of Serbian soldiers, most are barefoot, captured at Kreka, near Tuzla, Austria-Hungary (now Bosnia and Herzegovina). The Serbian Campaign of the First World War was fought from late July 1914, when Austria-Hungary invaded the Kingdom of Serbia at the outset of World War I, until the war's conclusion in November 1918. The front ranged from the Danube to southern Macedonia and back north again, involving forces from almost all of the combatants of the war. The Serbian Army declined severely 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 lost more than 1,100,000 inhabitants during the war (both army and civilian losses), which represented over 27% of its overall population and 60% of its male population. A prisoner of war is a person, whether combatant or non-combatant, who is held in custody by an enemy power during or immediately after an armed conflict. During WWI, about 8 million men surrendered and were held in POW camps until the war ended. All nations pledged to follow the Hague rules on fair treatment of prisoners of war, and in general the POWs had a much higher survival rate than their peers who were not captured. Individual surrenders were uncommon; usually a large unit surrendered all its men.