Our strategy "steel, not flesh" and it was a damn fine strategy. German generals made fun of American officers for not using tactics to take a strongpoint, but called in airpower. The simple fact is they were jealous, our casualties were far lighter because of that.
Russia didn't have that ability, her army was poorly trained and underequipped. Sadly, they still fight with "flesh, not steel" today. For whatever reason, Germany never realized how many Russians could be put into the army. Hitler and some of the top staff believed the encirclements of 1941 had defeated Russia. Some knew better, in The Allies Strike Back, Holland mentions a few commanders toward the end of '41 who went to Hitler and urged peace because they knew they no longer could win. But they were the rare breed, nor did they stay in their commands. The German army had no chance. I don't know why Hitler believed conquering Moscow would end the war, it did Napoleon no good to take Moscow. Why would it be any different for Germany? Maybe, just maybe, entering as liberators, they could have. But coming as conquerors, I don't know any way they knock Russia out.
Had Russia somehow folded, our ability to land at Normandy would have been very poor. I don't know if we could have fought through the mountains from Italy, but I doubt it. That's why I wouldn't say we had the greatest contribution in Europe, Germany's war was always going to be lost in Russia.