The bulk of Americans have a distorted view of what America was like during World War 2. While yes America eventually banded together to support the war effort initially that was far from the case. In the early 1940s most of America, FDR included didn't take the Japanese threat seriously. FDR himself viewed the Japanese as intellectually inferior and unable to shoot straight to his perceived notion that all Japanese were shortsighted.
In 1941 Hawaii was not a state and considered to be nothing more than a distant outpost. Even after Pearl Harbor most Americans felt the declaration of war unnecessary. Although Germany declared war on the US two days after Pearl Harbor, it was really nothing more than a symbolic raving of a mad man. Hitler's armies were bogged down in a dogfight with the Russians and the unexpected resistance. Hitler could not have mounted any serious attack against American soil. American troops did not begin to fight along the Brits and Russians until the end of 1942.
As 1942 was rung in the soon rationing and drafting of young men made the war even more unpopular. So concerned was FDR, his staff and the War Department FDR restarted his fire side chats. Famous and popular writer Alistair Cooke was sent across the country on a road show to sell the war to ordinary Americans. In a later book he described how factory owners were angry that the War Department took over production. Moreover, the prime demographic for factory and mine work, young males ages 18 to 35 were being drafted into service. Word came down that labor shortages would not be tolerated as an excuse for missed production quotas. Business reluctantly hired older men and women to get the job done and they performed admirably.
By 1943 Americans were for the most part supporting the war. However, a few were still critical of the war. Their primary objection was that the Brits and particularly the Russians were no better than the Germans. Even Vice President Henry Wallace publicly denounced British imperialism angering Churchill. FDR had no choice but to pull Wallace off the war effort.
By late 1943 the allies realized that to win the war they must invade mainland Europe as a path to march all the way to Berlin. From this came D Day. Planning for D Day feel mostly on the shoulders of FDR and Eisenhower. It was a complex, difficult, unsure, confusing and nerve racking process with risks galore. By 1944 FDR's health was failing. War Department brass met to consider options but a President Wallace was no option given Churchill's intense dislike for the man. Ultimately the word came down to FDR's doctors to keep him well enough to remain in the game of planning for the event. D Day by most accounts was a huge success and allowed the allies to eventually win the war.
By the summer of 1944 and after D Day in June FDR's was failing fast. He was ambivalent about a 4th term but said that if the American people chose him he would serve. He did next to no campaigning. Conservative Democrats at the time realized that FDR would never live out the term. A President Wallace to them could not happen. At the convention they forced FDR to take the benign and malleable Harry Truman as VP pick. FDR was too weak to put up his typical fight for what he wanted.
Although FDR accepted Truman as his running mate he never had any intention of allowing Truman to be a real part of his cabinet. Up until FDR died Truman was never invited into any White House, policy or planning meeting. He considered Truman a total twit. Truman wasn't told about the Manhattan Project or contingency plans being made with Churchill and Stalin for a post WW2 world. In fact during the entire time FDR and Truman only met once for a hasty lunch for PR purposes in which nothing was discussed. When FDR died Truman was totally unprepared for the job.
If FDR has lived any appreciable time during his 4th term we would not have had the Cold War. Stalin greatly admired and respected FDR. Both men were warm towards each other and had discussed a post war world in which all participants would become friendly countries. When Truman met Stalin both men instantly disliked each other. Truman further infuriated Stalin when asked by the press his opinion of Stalin calling him "a little odd fellow." Stalin never trusted Truman.
The myths of WW2 and FDR have been promoted for years but in truth both were far more complicated and varied. FDR had great capacity for compassion yet could be insanely cruel. There's never been a President like FDR and highly likely never again.