The combined Austro-Hungarian, German and Bulgarian offensive of October 1915 broke the Serbian front, and rather than surrender the army, the government and a large body of civilians withdrew across the mountains of Albania and Montenegro in winter. Bjelajac describes an ordeal in snow without food, clothing or medical supplies which killed large numbers of soldiers and civilians before the survivors reached the Adriatic; about 169,828 people were evacuated by Allied shipping, most to Corfu, where many more died. Serbia itself was placed under Austro-Hungarian and Bulgarian occupation, with internment of civilians in camps, requisitioning, and, in the Bulgarian zone, a campaign of forcible assimilation and the violent suppression of the Toplica rising in 1917. Bjelajac gives Serbian military losses including 79,000-81,000 dead in captivity, and states that civilian casualties of war and occupation have never been fully established, running to hundreds of thousands from epidemics, famine, camps and reprisals. The Serbian male population fell by between 17 and 30 percent across counties by the 1921 census.
Austro-Hungarian, German and Bulgarian forces overran Serbia in autumn 1915; the Serbian army, government and large numbers of civilians retreated across the Albanian mountains to the Adriatic, and the occupiers interned civilians and requisitioned food
Serbia's census of 1910 recorded about 2.9 million in the pre-1912 kingdom; the addition of Vardar Macedonia and other Balkan War gains brought the total to roughly 4.5 million by 1914. Serbia's wartime losses, military and civilian, are usually put at 1-1.25 million, among the highest proportional losses of the war.
Serbia's total war losses, military and civilian, are usually put at between 300,000 and over 1 million out of a pre-war population of about 4.5 million, the widest proportional loss of any belligerent. Bjelajac gives 29,249 killed in action, 43,027 dead of wounds and disease in hospital and 79,000-81,214 dead in captivity, and states civilian losses from epidemic, famine, camps and reprisals have never been fully established. The lower bound counts documented military and epidemic deaths; the higher includes contested estimates of civilian mortality.
Reprisal killings of civilians in 1914, internment of Serbian civilians by the occupying powers, requisitioning and the Bulgarian assimilation campaign are documented in Austro-Hungarian and Serbian records and in the neutral expert reports summarised by Bjelajac.
Kosovo, Albania