What are some conspiracy theories you absolutely believe to be true?

I don't see the US government perpetrating the attack. I could see the US government getting good evidence it was going to happen, and just... letting it happen.

The former requires a whole bunch of people involved that would need to keep their mouth shut about the things they did. The latter can be completed by simply not doing anything (someone in charge saying someone else will look into it, treat it like a hoax, or just file it somewhere and otherwise do nothing). If they wanted some major attack to go through, a couple loyal (to the agency) sociopaths people already used to working in highly confidential operations could drop some suggestions and assistance into a terrorist cell and then sit back. It's possible and knowing what we know about the government is not outside the realm of reason. Still, with no evidence of it, it is purely speculation, and believing it is an enormous leap from it being treated as a thought exercise.

The conspiracy theories that involve a bunch of workers installing explosives through the towers, etc, are unnecessarily complicated and implausible.