Another interesting 'what if' scenario comes forth if the Germans had won and left the North Sea and Channel dominated by the Kriegsmarine (since airforces was of no consequences for navies yet)
Sadly, it became a war of attrition to see who could kill off the other sides generation of young men first. The US impact was mostly the moral boost, which convinced friend and foe that only one outcome was possible now. Having bled France, Belgium, UK, Austria-Hungary & Germany dry, a new player with an untapped source of not yet dead young men was a major shift in the balance of resources.