Nazi scientists moving to the USA after the WW2 is in no way a conspiracy - it happened and the US didn't "turn a blind eye", they actively encouraged it.
The space programme built on the V2 rockets that smashed up London in 1945.
As for the rest of the theories on here, I think they're all nonsense. Although I am currently reading a book where the premise is that the 9/11 attacks were engineered by companies that would do well out of wars in the Middle East. That does not seem far fetched and would explain odd behaviour on the stock market.
My personal favourite conspiracy is that Amazon started the pandemic so that everyone would buy stuff from them online.