Which country had the most casualties during WWII and how many?

What MKB said is correct in the sense of the mass of casualties. However, if one were to look at the percentage lost, Poland would have to top the list. During the Second World War, nearly twenty percent of Poland's population was slaughtered as a result of war efforts and the persecution of Jews, other religious groups, homosexuals, and anyone else who attempted to interfere in the Nazi regime or was considered inferior. That's almost one out of every five citizens, and such devastation equaled roughly six million people for what Poland's population had been at the time.