Hitler clearly underestimated the military power and might of the Soviet Union, despite that his primary strategic fault was that he opened multiple fronts at the same time and he continuously added new enemies to the already existent, no army in the world could be able to sustain a prolongued warfare in multiple fronts for a long time and Hitler simply didnt know where and how to stop his risky and according to others (insane) military venture.
Although he delivered a blow to the mighty British Empire and he bombed London and several major cities of England he couldnt prevail in the air because his generals and especially Goering who was the chief commander of the Luftwafe, didnt bomb the British airports when Luftwaffe had airsuperiority, at the beggining of the war and during Dunkerque evacuation which is considered to be another major mistake on behalf of Goering since most of the British expedition force and many french soldiers managed to escape intact in England.
Therefore the fact is that Hitler didnt have airsuperority and despite the success owed mainly to his U-Boats who indeed were devastating the British convoys his main battle ships could not match the British except Bismark which was sunk by the British a major loss that its concequences became apparent later on in the war, therefore although Hitler delivered major military blows to England nevertheless he never managed to completely defeat and despite this fact he choose to open a new front with the Soviet Union
If we assume that Hitler had prevailed in the war against England he would aparently had faced many losses especially concerning his airforce and his navy.
So he would have less available forces to use in operation Barbarossa and after he had finished operation Barbarossa succesfully his forces would be almost decimated due to the heavy losses that his army would have to endure
after all Russia is a vast country with a big population and the red army was very strong especially concerning its armour units (around 40,000 tanks) and artillery, therefore even with the most optimistic scenario Hitler would face such big losses that it would be almost impossible for him to succesfully continue the war against USA.
We should not also forget the operations in Africa (Africa Corps) and the troops that he defenetely needed in order to protect the occupied territories in Europe, America wouldnt face an immediate danger coming from Germany for many reasons but as paradox as it may sound the main enemy threatening America directly this period of time was Japan.
This is what happens when you multi-multi-task.
