According to the National WWII Museum, the Soviet Union suffered the most casualties in World War II. There was approximately 8 million to 10 million military deaths, and 24 million military and civilian deaths.
The six-year conflict is considered the most destructive war in history. All death tolls are an estimation, as there is no definitive way to know how many civilians were killed in the war. Behind the Soviet Union in number of casualties was Germany, China and Japan, each suffering death tolls in the millions. While this was a military conflict among many countries, the majority of the deaths were often civilians.