I haven’t really studied the Civil War so I can’t speak to that ; but the reason German (and Japanese) leaders thought they’d get away with it is because they were fascists ; and fascists are implicitly, inherently incapable of correctly estimating the strength of their bugaboos.
It’s true of every fascist group, but probably even more marked in those two cases because so much of their ideologies were racist/based on pop-genetics on top of being fascist. And that’s absolutely dysfunctional and schyzo at its very core because on the one hand, you can’t be a fascist (or really an extremist of any persuasion) without an enemy to rail against. In the case of the Reich, it was communists and Jews (mostly) which were, in their opinion, such a dire and existential threat to Germany that they had to be destroyed ASAP. Every extremist policy, every “national purge”, every robbing of liberty, every new law, every oversight on violence by the Nazis was invented and presented as the only possible effective answer to the twin headed, all-powerful, all-controlling hydra of Judeo-bolshevism which the Nazis were the one and only rampart against.
That’s column A.
But in column B you have all the Aryan bullshit and the inherent superiority of the German race and culture over everything else ; especially the deviant less-evolved parasites that were Jews and Slavs. They weren’t even human ! Human-shaped rats, cockroaches, lice on the world ! So how could they have possibly resisted the strong pure might of a proud German soldier kicking the door in, newly purged of any of his past social-democrat flaws and infused with a glorification of strength and brutality ? *Unmöglisch *!
The implicit contradiction led them to the notion that there was a war that absolutely needed to be fought immediately else Germany would be destroyed ; by an enemy that was, somehow, both perpetually historically victorious but also implicitly and inherently weak, lame, inferior etc etc… And if you think that’s weird and dumb, hi, these are the Nazis, perhaps you’ve never met ? ;).
Turns out doublethink is not conducive to clear thought.
They didn’t even think the US would do that to begin with.
Hitler was extremely surprised (and much angered) that the US didn’t support him ; he’d got it in his silly head that the two countries were natural allies. And to be fair, eugenics and antisemitism were not exactly rare in the US back then. Naturally, when it turned out that Roosevelt wouldn’t play ball, he turned around and accused him (and America) of having been Jewish/Freemason puppets all along. Because that’s the only thing that could possibly explain such an illogical turn of events. Perfectly clear. Not a single doubt. The Nazi theory of history and the world was implicitly correct, therefore it’s reality itself that must have been perverted and Wrong.
Similarly, he was adamantly convinced that the British, being a) democrats ptooie and b) not quite untermenschen necessarily would rapidly cave to his aggression, understand why he did what he needed to do, appease him and let him concentrate on destroying Stalin because after all, it was in their interest too, ja ? They, too, were threatened by the hydra of Judeo-bolshevism, that much was perfectly obvious ! And Hitler didn’t even want to exterminate them ! So to keep fighting like they did was, * in his own words*, “contrary to all logic and necessity”.
So yes. He really believed he could do whatever he wanted and the UK & US would let him, help him even. He really was that blinkered.