Nobody but idiots who grew up on Si-Fi stories is prepared for WW3. Nobody wins a (world) nuclear war. Lefties and their minions in the media didn't seem to mind the scariest scenario of a mentally impaired president with his trembling thumb on the nuclear button. At least Trump gets credit for stopping half a dozen brushfire wars.
Thank you. You just answered your own question. Most of American "normies" (not only FDR administration) were not prepared to WWII because they didn't believe it's possible. "What is, in God's sake, reason to make preparations to something that will never happens, that simply can't happen"?
And no, I'm a big fan not only of Sci-Fi, but, say, also of military and history books. And yes, history doesn't teach anyone mostly because everyone believes that "today is not what was yesterday, today's world is totally different". And you know what? They are always wrong. As was wrong W.D. Puleston (real naval expert), who wrote in The Armed Forces of the Pacific, 1941 (pp 116-117):
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The greatest danger from Japan, a surprise attack on the unguarded Pacific Fleet, lying at anchor in San Pedro Harbor, under peacetime conditions, has already been averted. The Pacific Fleet is at one of the strongest bases in the world - Pearl Harbour - practically on a war footing and under a war regime. There will be no American Port Arthur.
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They didn't believe in "American Port Arthur" and they got it. Not in San Pedro Harbor, but in Pearl Harbour.