If the Germans stationed all the manpower into France and Italy that they wasted in Russia, the Allies might never have invaded either country. Likely, they might have just waited for the Atomic bomb to be developed, then that would have been that.
They could have taken the UK and Ireland easily with what they had. That would leave the US without a toehold anywhere in Europe. Then Hitler would only have had to sit tight and hold parades and rallies.
However, that is not the sort of thing that megalomaniacs tend to do....