A couple of things you're missing:
- Hitler had rich German backers who specifically wanted to destroy the Weimar Republic[0], because democracy was starting to turn on the German capitalist class.
- Hitler was never popular enough to gain control through democratic means. The Weimar Republic was split in thirds between liberals, Nazis, and communists; the liberals thought letting Hitler be "vice-chancellor" (under a liberal chancellor) would be the least bad option. Hitler exploited this and demanded the chancellorship at the last minute. Once he was in position he was able to cause chaos and rip up the Weimar government.
- Very similar events played out in America.
America had its own very popular fascist parties. Furthermore, we had a very long history of people wanting to subvert or exit democracy in the name of white supremacy[1], and even a successful Presidential assassination to stop the fledgling Republican Party from stopping the South from reinventing slavery. Our constitutional guardrails are actually really thin and always have been.
1930s America also had very similar economic problems to Germany. We didn't have crippling war debt or hyperinflation, but the Great Depression was a globalized problem, so everyone had people demanding a strongman, which means fascists have a stall in the marketplace of ideas.
FDR was able to avert catastrophe, largely by subverting several of America's constitutional defenses against dictatorship. To be clear, capitalists had already coopted and corrupted classical liberalism, and they were able to successfully get the Supreme Court to shut down every moderately Progressive[2] policy because the one thing the Constitution was good at stopping was those policies. FDR threatened to pack the courts, and then suddenly the Supreme Court shut up.
But before that, the American capitalists tried doing exactly the same thing Hitler's backers tried - hiring a strongman to go and take over the US government[3]. Except they hired Smedley Butler, who was already getting tired of being Wall Street's hitman, so he immediately blabbed about it to the government. I'm under no illusion that America had plenty of competent men who would sell their country out in order to sit on a comfy chair and let the capitalists loot America. We're just lucky the capitalists picked the wrong guy.
Ultimately the thing that got America out of the Great Depression was WWII - and not because wars are inherently good, but because it gave FDR a blank check to rebuild the economy with government money. And yes, FDR had to engineer this too, by embargoing Japan and daring them to attack us. And yes, even with a not-shitheaded liberal running the show there were still dramatic overreaches of government power[4] that our constitutional guardrails did jack shit against[5].
Not to mention the whole "running for four terms" thing. Yeah, that's right, Presidential term limits were a norm - not a rule - until FDR decided he was just going to keep going until his body stopped him.
America did not come out of WWII with its democracy intact because it has superior structures. Nor because its people are inherently more trustworthy or we had more experience with democracy. (I mean, we did, but barely.) It was largely dumb luck:
- Luck that America's fascist movements didn't shoot first.
- Luck that the Progressive movement backed a liberal, not an authoritarian. FDR absolutely had all the power and could have destroyed American democracy instead of rebuilding it.
- Luck that the capitalist reaction stumbled at the starting line. The Business Plot could have taken him like the South took Lincoln.
The only thing mostly determined at the outset was that we were going to win the war, because we owned the oil. That's why America is still obsessed with oil to this day.
also
>Hitler's military did well in fast attacks against opponents not yet taking war seriously.
This is an echo of how authoritarians take power: do something so batshit insane so quickly that nobody has time to notice you palming everyone's phones. Think like January 6th: had Trump actually been coordinated rather than just angrily lashing out, he could have actually stopped the election before it was certified, gotten his 6-3 Supreme Court to look the other way, and then seized power.
[0] There are historical echoes to the French aristocracy's attempts to choke fledgling democracy out, though in that case the fledgling democracy went paranoid and made its own dictators first.
[1] To be clear, "White" was far narrower then than it is today. It excluded the Irish, Mormons, Italians, and so on. But for the purpose of this discussion we can use the modern colorist definition rather than the far more racist definition they used back then.
[2] As in the political movement, not the extremely genki insurance salesperson character
[3] https://en.wikipedia.org/wiki/Business_Plot
[4] https://en.wikipedia.org/wiki/Internment_of_Japanese_America...
[5] https://en.wikipedia.org/wiki/Korematsu_v._United_States