Military defeat does not require total conquest

Does failure to conquer the Soviet Union mean an Allied war against it could not be won?

No; a state can be defeated without total occupation, and success or failure should be measured against its war aims.

Military defeat does not require the complete conquest of a state. Imperial Germany defeated Russia in the First World War without occupying the entire country, so an Allied victory over the Soviet Union would not require total territorial control. The same test applies to wars of aggression: an attacker that fails to achieve its aims and is forced back from invaded territory can be losing even while the fighting remains outside its homeland. Russia's invasion of Ukraine supplies the contemporary example, though the final outcome is left open. Victory should be judged against war aims and campaign direction rather than a simplistic map test.