Where the hell do you get your history "knowledge" from?
Where does that "US invests in Germany" story come from? The US propped its economy up for another ten years with the money flowing in from Europe and the shit hit the fan a decade belatedly, but investment certainly was not the name of the game of the time. The money was mostly used to fund the bubble that popped in 1929 because there was simply nobody abroad that could actually serve as a demand for all the junk the US pumped out (in case this sounds familiar, well, history repeats itself).
But that was not even the main reason for the rise of the national socialists. The economy crisis itself (that started pretty much right after the war in Germany and most of the rest of Europe to a lesser degree) was even a minor reason. The main reason was the feeling of unfair treatment and the thirst for revenge.
George Clemeceau was the driving force behind the "crippling" of Germany. His idea was that a Germany that cannot wage war will secure France's eastern border. So his goal for the peace between Germany and France after WW1 was to ruin Germany. On the outside, that plan is solid: A country with no money, no political power and no military power is no threat.
What he didn't take into account was that a country that you abuse to the point of breaking will resist this treatment. Especially when the general feeling is that this treatment is not deserved.
The first reason for this was the front line at the end of WW1. When you look at the front line between Germany and France, you will notice that by the time the armistice was called, the front line was actually well within the territory of France. From the point of view of a German soldier, there was no obvious reason that they lost. Hell, we won territory! We ain't the losers here! And certainly not losers that deserve to be crippled in such a way!
Well, you also should take into account that a century ago, waging war was not an "evil" thing. War was, quite literally, just politics with other means. And it was seen as such. Wars also never had that kind of dimension before. War was something where two countries fight, after a while they settle, some territory changes hands and everyone moves on. That's what wars were like 'til then. The idea that wars end governments was pretty new then. But this just as a side note.
The army still standing rather deep in enemy territory when the armistice was signed and the "unfair" treatment by the French quickly led to the Dolchstosslegende, the myth that the German army was not really beaten but that it was assassinated by a stab in the back by ... well, insert you favorite internal enemy here. Jews, socialists, old government, pick your favorite scapegoat.
Combine this now with a peace that doesn't aim at peace but at crippling the country losing the battle and you probably find out why this is a breeding ground for radical ideologies. And we learned our lesson here. Any country you try to neutralize by ruining it will do anything to shake off those shackles. No matter the cost.
