World War Two, USSR: at least 20 million in a population of 205 million (https://en.wikipedia.org/wiki/World_War_II_casualties_of_the...), or close to 10%.
World War Two, Poland: about 6 million deaths (https://en.wikipedia.org/wiki/World_War_II_casualties_of_Pol...) in a population of about 35 million (http://countrystudies.us/poland/27.htm) or over 17%.
The US civil war was bloody for the USA, but not bloodier than WWII for any Allied power.
https://en.wikipedia.org/wiki/World_War_II_casualties#Total_... has many more countries with a death rate over 2½%.
World War Two, USSR: at least 20 million in a population of 205 million (https://en.wikipedia.org/wiki/World_War_II_casualties_of_the...), or close to 10%.
World War Two, Poland: about 6 million deaths (https://en.wikipedia.org/wiki/World_War_II_casualties_of_Pol...) in a population of about 35 million (http://countrystudies.us/poland/27.htm) or over 17%.
The US civil war was bloody for the USA, but not bloodier than WWII for any Allied power.
https://en.wikipedia.org/wiki/World_War_II_casualties#Total_... has many more countries with a death rate over 2½%.