the us claimed that Iraq had wmds, but it was later revealed Iraq has no usable wmds (they had some sarin that expired in the 70s) and the us had a congressional hearing where the armed services confirmed that they considered the unusable wmds as active wmds, even though they could not be used. it was not a conspiracy later proven true like the Tonkin incident, but just using the vague definition of WMD (there is no offical one delcared by the UN) to the army's advantage

The justification for invading Iraq was a claim that they were developing nuclear weapons. It was well known at the time that the evidence was flimsy, and that even if true it was a flimsy excuse for an invasion. The main piece of evidence was an intercepted shipment of aluminum tubes that were soon shown to have nothing to do with a nuclear program. (See https://en.m.wikipedia.org/wiki/Iraqi_aluminum_tubes). That one is not a conspiracy theory.

Honestly, it's much easier to come up with a reasonable conspiracy theory around 9/11 than most people think. There's proof that the FBI and DHS knew that the attacks were being planned (iirc they were warned multiple times by Mossad), so it isn't that much of a stretch to believe that the US intentionally ignored the warnings with the knowledge that such an attack would justify another war in the Middle East.

Ignoring the warnings would be a win-win. You get to go to war for oil if they're real, and if they're fake, then nothing happens and life goes on like it always has.