The number of Russians who died in World War II is undetermined, but estimates range from 20 million to 26 million. Even the low end of the range represents more deaths than the combined numbers suffered by Germany and Japan.
Stalin's strategic missteps and political ruthlessness were the main causes of Russia's heavy military casualties. His political purges eliminated the army's best generals, and he positioned his troops along the Polish border instead of garrisoning them in their fortifications inside Russia. Stalin ignored warnings about the advancing Germans and refused to mobilize his army. When Hitler attacked, the Russian army was defenseless with incompetent leaders in place. Stalin was also insensitive to Russian casualties. He deliberately sacrificed units designated as "punishment battalions" and pressured his generals into achieving symbolic victories at the expense of their troops.
Scale of Warfare
The sheer scale of the fighting was also a critical factor. The battles on the Eastern Front were the largest battles waged in military history. Hitler's invasion force consisted of more than 3 million soldiers, 3,500 tanks, 1,800 planes, and 7,000 cannons. More than 1 million Axis soldiers fought in Stalingrad, and more than 5,000 aircraft and 8,000 Russian and German tanks waged war in the Battle of Kursk.
The final factor was Hitler's genocidal war policies. Hitler considered Slavic Russians to be sub-human, and he ordered the Einsatzgruppen to exterminate them. Estimates indicate the Nazis executed 7 million to 14 million Russian civilians during the war.