World War II was the most destructive war in history, costing millions of lives and severe damage across many areas. The war involved battles across numerous countries that killed over 1.6 million people at Stalingrad alone. Russia suffered the most casualties at 21 million total. The genocide of European Jews and others during the Holocaust involved the systematic murder of Jews, initially by shooting but later in gas chambers to more efficiently kill victims at places like Auschwitz, the largest concentration camp network.