US civil war: about 750.000 in a population of over 30 million (https://www.history.com/news/civil-war-deadlier-than-previou...), or 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%.