Who lost the most in WW2?

In terms of sheer civilian casualties, the Soviet Union lost the most. The Soviet Union lost around 27 million people during the war, including 8.7 million military and 19 million civilians. 

European Jews suffered the most in terms of demographics. The European Jewish population before the outbreak of World War 2 was estimated to be 9 million, an estimated 57% of the world Jewish population. 6 million would die in the Holocaust, a policy of systematic extermination carried out by the Nazis and their supporters.

The Holocaust was carried through a network of concentration camps that were set up in territories occupied by the Nazis. Most of the concentration camps were in Poland. It was also where most of Europe’s Jews lived.

For further reading check the following articles:

• World War I
• World War II
• Causes of World War I