AAQ.
Firstly, I believe that lend lease was mainly an anglo thing. Not tonsure the us aided the Slavs.
But assume for a sec they did; this aid had nothing to do was pearl harbor. America was isolationist bit only insofar as sending troops. Not vis a vis aid.
The point remains that Russia would probably have destroyed the German army and conquered Germany wo any american boots on the ground.
Zog ich, the main reason the Americans and English launched D-Day etc. was not to stop the Germans but to prevent communism from conquering Europe.