I have to agree with Free, Europe was the training ground for deployment and us of our military in WW II. The stage was set in Europe. The blunders of Hitler lost the war for the German Military. I believe Hitler would have been defeated in the long run, but the advance on Russia was so poorly timed and conducted. Stalin's push west was unstopable. No amount of diplomacy at Yalta would have called it of; or blunted Stalin. Even if FDR was at his best, he could not have talked Stalin out of what happened. That said, FDR's health while @ Yalta is more debate fodder than anything else.
I have to agree that FDR's decisions were sound. His decisions regarding what military leaders were also spot on, in the end. The placement of key military advisors and leaders was also war winning. Not so much so for the Brits. Montgomery, Patton's nemises, was probably a good field commander. He just made too many blunders in Africa, and with Market Garden, to retain the position he had. Ike was not going to sack Monty, and was stuck with him to the end. In that end, I think the Brits finally figured out Montgomery's worth; try to find his statue in London.
The war in Europe, as mentioned above, trained us to use our military with great results. I agree that Japan was seen by the US as Stalin's Germany. Japan was going to pay for Pearl Harbor, it was only a matter of time. It would be interesting to know FDR's plan for employing the Atomic Bomb.