If an alien race came down and abducted you and said, "We'll share with you all our intergalactic secrets...but only if you let us touch you...down there", would you do it?
The economic and manpower injection helped suffocate the Nazi jackboot, but let's not exaggerate. America had it's own interest in entering the war, and lets not forget she was happily funding the strong economic Germany of the 1930s.
WWII was won by a demagogic dictatorship that had no sustainability, it either kept attacking and growing or it died. The Allies basically fought to make sure it couldn't grow so fast.