There's no doubt that the Russians were the main opponent of the Germans in WWII. The entire allied force on the western front was a fraction of what Russia put together. The Russians had almost as many soldiers killed in one day than the Americans did in the entire war including the ones lost fighting against Japan. At Kursk the Russians had 250,000 killed and another 600,000 wounded. That dwarfs the entire US involvement in the war. The Germans had 100,000 killed that day too BTW.
The US had about 290,000 killed in the entire war with about 670,000 wounded. Think about it. The Russians had almost that many casualties ON ONE DAY! People who think the US was the decisive factor in the war just don't know the story. In fact the US was barely a blip on the radar screen as far as the actual fighting went in Europe. It's no wonder the Russians got ticked when it came time to divide up Germany after the war. The western Allies got 3/4 of the pie but only did about 10% or so of the fighting. I'd be ticked if I was running Russia.
The second front helped as did the N. Africa campaign as did the Italy campaign and the southern France invasion. But by far the Russians bore the brunt of the fighting in WWII in Europe. It's not even close.
The US did bear a big part of the fight against Japan. But there were lots of other countries helping. And the fact is the US dominated Japan after Midway. Japan lost far more soldiers than the US did. They had almost a million more deaths than the US had and many of the US deaths were in Europe. Japan only had about 140,000 wounded which tells you a lot about the Japanese attitude toward life. The US controlled the air after Midway and espeically after the Marianas Turkey Shoot. The US faced green troops and the only real threat from the air was the kamikazes. The war was lost but Japan just refused to lay down and die. So they threw a lot of men to the lions and we killed them. They kept expecting another miracle like the one that stopped the Mongolian invasion of Japan. A cyclone stopped that monster army from taking Japan and the people of Japan expected to see another miracle. They saw a divine wind alright. But it wasn't blowing the right way for them. The shock wave from an atomic blast is a terrible wind indeed.
Also the idea that the Germans weren't effective if they lost their commanders is largely a myth that was created as propaganda during the war. The chain of command was followed closely by German troops so they pretty much always knew who was in charge. And the commanders in the field, even if it got down to a Sgt., had a lot of leeway in how they fought.