I am no WWII expert, by any stretch. I have a friend who is, who teaches history at a college, designed an intricate WWII table game and has played the others in tournament for decades, and just spent two days at the New Orleans museum. I note this last bit because I could not spend two focused days at any museum, not even one devoted to what a wonderful guy I have always been. I can see doing a morning and a following afternoon, but not two opening-to-closing days. I can see getting a membership and coming back often. But you have to really love something to pound it in like that. Perhaps I only say that because I usually go to small, obscure museums that can't put the bang into the displays that a larger, national one can. But I spent an afternoon at the WWI museum in Kansas City and thought my brain was full and could take no more.
I am not in his league. Among amateur historians in America, WWII is by far the most popular branch of study, with the War Between The States a distant second. WWI and the Revolution don't come close. This applies to alt-history and gaming as well as serious study. There are a lot of eyes on this information, and a lot of discussion. Nonetheless, I do know a bit, and as a discussion of Roosevelt started up in the comments under The Other David Wyman, I thought I would give it some air and weigh in myself, not comprehensively, but provocatively. I don't in the least advocate this is what we should have done, but interesting to consider in light of any "Did Roosevelt get it mostly right?" discussion. I think Pat Buchanan does argue for something like this, but I haven't read it.
Hoover originally did not believe we would need to go to war, and that Churchill and others were unnecessarily provocative so that Western Europe would get dragged in. His vision was that Hitler and Stalin should slug it out without us. It would have perhaps been possible to make reassurances so that he did keep his focus in the East. I don't dismiss this as impossible. There was a lot more Lebensraum in Russia than there was in the Netherlands. After they had destroyed each other, an untouched Western Europe could have imposed its will on both. If that seems irresponsible and cruel of us WRT the Jews, it pays to remember that things could hardly have turned out worse than they eventually did, and the Final Solution might not have been enacted if there were no West threatening Germany. One can even imagine a scenario in which America and NW Europe focused on rescue of Jews, which Hitler was only too happy to be rid of, in any direction. I call that less than 50% likely, but not impossible.
Several things make this whole train of speculation unlikely to have happened, or to have worked. First, if Japan had still attacked under this scenario, the war of some sort with Germany would have been impossible to avoid. However, that attack is not a given, certainly not in a situation where even England and France are not formally at war with the Axis. Pearl Harbor was calculated to discourage us from even thinking of entering the war, because we clearly were thinking of it.
Secondly, it might indeed be irresponsible to abandon the Jews that fully. Though again, how much worse do we think things would have been. Absent the natural "but we have to do something" feelings, would doing nothing have totaled more than six million Jews plus whatever the Soviets added? In this imagined alternative, we don't have to rescue or protect them all in order to get a better outcome. I do notice it would also involve abandoning the Poles and the Czechs right from the start. Worse outcome for them? We set the precedent when we intervened in WWI that we were not entirely indifferent to Europe's, especially England's, wars. (Tangentially, if the Spanish Flu did in fact originate in Kansas, spread to Europe by American troops with devastating effect, one wonders how much good we accomplished.)
Thirdly, there is still the problem of oceans, and especially the North Atlantic. Both Germany and Russia needed freedom to move ships that runs right through seas that the UK, Scandinavia, and France had a lot of say in. Staying out of war may have been impossible for them, and thus less possible for us, trying to supply them.
Fourth, we have no assurance how "letting them slug it out" would actually play out. That we can imagine something as very likely does not mean it is likely. Germany and Japan slugging it out with Russia and China does not suggest stability to me.