During World War II, which Allied nation suffered the most military deaths?
Russia, actually it was known then as USSR.
A comprehensive study done in 1993 by the Russian Academy of Sciences estimated total Soviet population losses in World War II amounted to 26.6 million, of which 8.7 million were combatants. That is equal the entire population of Nepal or the entire population of Texas.
The German forces during the invasion of the Soviet Union utilized a campaign of terror. Leningrad was surrounded and starved in a siege. Despite terrible loses of civilian population the city never fell to the German forces.
Stalingrad was a climatic battle during world war II. The entire city was leveled during the fighting. The civilian population was bombed and shelled indiscriminately along with military targets.
The civilians in Jewish centers were rounded up and shot and buried in mass graves. Any town that resisted the German invasion or gave aide to the partisan resistance fighters faced retaliation in systematic executions of the civilian population.
The Russian army in the beginning of the War was poorly led. Large formations of Russian infantry were surrounded and destroyed. Russian prisoners were not well treated and few survived after being captured.
All of these factors contributed to the terrible loss of life in the Soviet Union during World War II