What are some notable blunders in history that resulted in huge loss?

Kinda. The winners of WWI decided to leave Germany be, but took most things of worth and some land. The reparations were brutal. I think Germany finished paying of the reparations a few years ago. Additionally there was military propaganda that the reich was "undefeated in battle, stabbed in the back", because the civilians negotiated the harsh peace treaty and ignoring the fact, that the war was going badly.

I will not go more into details because I do not know exactly. But the combination of a very depressed economy, the feeling of being treated unjust and the desire for revenge led to a disgruntlement -> rise of populism -> rise of extremist parties.

I am missing a ton, but when things are unstable it is easierfor radical forces to emerge and succeed.

Hitler literally was tasked to spy on the NSDAP and joined them. You have to see: at the time two major parties in the reich were anti constitutional.

That is why a lot of people in Europe look worringly at trump or at least at the whole movement. The USA has issues that need fixing. There is a large disgruntled part of the population and people start to radicalize.

I may generalize, but the start of WWI was mostly a series of pride, miss communication and bad luck.

Nazism may have been the worst thing that ever happened to the German people.

but didn’t WWI help set the stage for the nazi party’s rise in Germany? Yes, but the Great Depression was another big factor. It amplified the country's economic woes...