The map below shows World War One casualties [civilian and military] as a percentage of a country’s population. The darker the colour the higher the percentage. The country with the highest national casualty rate was Serbia. France lost 16.8%, Germany 15.4%, Russia 11.5%, Italy 10.3% and USA 0.13%.
Serbia lost 27% of its overall population and 60% of its male population. A total of more than 1,100,000 people.
The nation responsible for these losses was the Hapsburg state of Austria-Hungary.
Austria-Hungary was not only responsible for the Serbian losses but, in the minds of many historians, for starting the First World War. Germany may have been a wiling accomplice but Austria-Hungary was the instigator [see Watson, A., 2014. Ring of Steel: Germany and Austria-Hungary at War, 1914-1918, London: Allen Lane.]. Austria-Hungary attacked Serbia [after giving them an ultimatum that they knew the Serbs could not accept]. Russia moved to assist Serbia and, thanks to a series of existing treaties France, Britain, Germany and Italy were dragged into the conflict.
Austria-Hungary invaded Serbia. As well as being swine they were also incompetent and initially they suffered heavy losses. However, Austria-Hungary had more men and guns and in time they were able to defeat the Serbian Army so completely that about 155,000 survivors had to be evacuated to Greece by British ships.
That left the Serbian population at the mercy of the Austro-Hungarians who carried out numerous atrocities. Concentration camps were set up, there were mass hangings, crops were destroyed and wells poisoned. In addition to all this Serbia suffered a severe typhoid epidemic.
At the end of WW2 some war criminals were executed. Things were different after the First World War. Count Berchtold, the Austro-Hungarian Foreign Minister, who was more responsible than any other for the start of WW1 was allowed to retire to his estates in Hungary where he lived until 1942.