There is nothing to be baffled by. You’re just misrepresenting the argument.

It baffles me how you don’t see the hypocrisy of both complaining about the US not joining WWII until they were directly attacked and also complaining about American hegemony today.

It’s only baffling if you assume ab initio that the only possible kind of intervention is the imperialist, interventionist one, and that that is the only way of describing the country’s (or any other Allied country for that matter) entry into WW2. More generally, its only baffling if you assume that involvement naturally equates to “hegemony”, and the behavior that implies, in the long-term. This viewpoint totally negates the normative side of power which is why it has been all but abondoned by contemporary IR scholars, political scientists, sociologists, etc.

In short, you misrepresent (deliberately or otherwise) your opponent’s argument by assuming that all exercise of power is “hegemonic”, an assertion that is not grounded in reality. At this point, you should also be able to see the moral issues with some of what you said and the overall image you presented of the human condition. Classical geopolitical thinking is simply not valid and tends to reproduce highly unstable and dangerous systems by ignorant human who reify it into reality.

Can you articulate why, with what they knew in 1939, the US should have declared war

Sure (and you too should be able to - its real simple). It starts with an f and with a ascism. Though I’ll give you that policy analysts at the USDOS at the time didn’t see it in those terms. I’m also willing to bet they knew a lot more than you think you know but do let me know if you think I’m wrong.