I’ve read the FDR would have left Britian to their own devises if Russia looked like it was going to be overrun
I don’t think that most people have any idea just how anti-war most Americans were going into WWII. Nor do I think people understand how anti-British many Americans were.
There were probably several reasons for the anti-British sentiment. We had fought Britain in two wars for national survival and had a tough time of it. People were still taught history then, and they thought of Britain as an aggressor. Globally, Britain was still a colonial power, and we were anti-colonial by birth right. Finally there were a lot of Irish immigrants that had absolutely no use for the British at all.
Even after WWI we were still making warplans to fight the British and defend our Eastern seaboard from the Royal Navy.
That is actually a sensible policy. Without an Eastern front the eventual invasion of a German controlled Europe would have been impossible. If the USSR had surrendered I think that Great Britain would have come to terms with Hitler.