Nazi Germany was such a large super power that it required both the USSR and the other Allies to work together to do this. Without the US and British forces in France, and Italy. The Germans would have had all resources to push unto the Russians and completely take them.
Until United States entered the ww2 Russia has lost many raids. Th e Nazis were in Stalingrad, before Moscow and have sieged Leningrad. US supported them with weapons. And just after USA entered the war, everything changed. Soviet Union would heve been defeated in months without the help of Allies.