I'm from England and my (simplified) knowledge of America's influence is the war is ' After WW1 Ford introduced a new car, this got people lots of jobs for lots of reasons, people starting buying more and more to achieve the American Dream, people got greedy, bought things they couldn't afford which launches a stock market crash and then a global depression. This lead to an increase in radicalism in Germany and people voted for the Nazi Party, WW2 was declared, at the end of the war America joined in and then declared victory, despite it already looking like Britain was about to win'
TL;DR Great depression caused radicalism in Germany, Nazi Party elected, WW2 happened, while Britain was winning America joined in and said it was their victory.
So what was America's real part? Did they actually just piggy back on Britain or with out them the war would've ended much differently?
You could say that the US joined the was after the Japanese bombed Pearl Harbor on December 7, 1941. This is certainly when the US actually committed troops the fight, but in reality the US had been supplying many tons of supplies to Britain and Russia in the form of cash and carry, and eventually lend-lease material. Cash and carry told Britain and Russia that the US was willing to sell them supplies like guns, fuel,and ammo, but that they must pay for it up-front, use their own ships to transport it back to their respective countries. President Roosevelt (who was adamant about getting America into the war) eventually convinced congress to pass the lend-lease act, which told the allied countries that they could take American weapons, ammo, and other supplies, and pay for them later.
When the US did send combat troops into the ETO (European Theater of Operations) and the Pacific theater; things were looking quite grim. Operation Barbarossa was underway and the Russians appeared to be totally on the back foot. Things in Britain were not much better; the Battle of Britain was a German failure, but the Blitz of London was in full swing, and it appeared that Germany was on the brink of total victory in Europe. That's not even mentioning the Pacific theater of Operations, which the US won almost single-handedly.
The US presence did a number of things; firstly, it provided the Brits with an overwhelming advantage in the air, as the Germans could simply never match the number of planes that the US could produce. It also gave the allies the quantity men and equipment it needed to secure northern Africa and Italy. The Brits simply didn't have enough men to win a ground war that it needed to. Another -smaller- aspect that can be looked at is tanks; the US supplied the the vast majority of tanks that the British used in WWII, the medium tank M3 (Lee/Grant) and more famous M4 Sherman.
In the end, I don't believe that Germany could have beaten Russia, even if they didn't have the US or GB to worry about. However, one can never truly be certain when discussing something as gargantuan as WWII. However, one thing that I am totally sure of, is that the US did play a huge role in WWII, and the outcome surly would have been different in many ways.
I hope that this odd rambling has in some way answered your question. It's pretty late and I didn't flesh the response out as much as I wanted to, but if you have any other questions, I would be more than happy to answer them.