I realized recently that a lot of people writing about it today were not necessarily old enough to follow politics yet at the time of the events.
No, the WMD claim by the US was not an honest mistake, it was a purposeful lie to be used as an excuse for war.
Here is a speech by GWB on Iraq in 2002: https://www.theguardian.com/world/2002/oct/07/usa.iraq
The claim was that Iraq threatened USA. At the time, everybody knew this was a lie. Iraq did not have capable missiles to reach US territory and claims that it had a secret nuclear program were extremely dubious. Remember : the fear was nuclear ICBM, this is what was hammered in the media. They fell back to chemical weapons, but if the US plans to invade every country with a stock of chemical weapons, good fucking luck!
Colin Powell, presenting the threats to the UN council, put forward a nuclear threat. He used as proofs, documents that were fake. "Obvious fake" as the head of the International Atomic Energy Agency would call them: http://edition.cnn.com/2003/US/03/14/sprj.irq.documents/
What he said about it: "My colleagues, every statement I make today is backed up by sources, solid sources. These are not assertions. What we're giving you are facts and conclusions based on solid intelligence."
These were not mistakes, these were lies:
https://www.huffingtonpost.com/jonathan-schwarz/colin-powell...
Iraq was (correctly it appears, in hindsight) fearing an invasion so it was worried that opening up to foreign inspectors would lead to espionage to prepare the invading forces.
In the end, as a last hope to preserve peace, it allowed inspectors in a mission, led by Hans Blix. Here is how he described the cooperation of Iraqi forces:
"Iraq has on the whole cooperated rather well so far with Unmovic in this field. The most important point to make is that access has been provided to all sites we have wanted to inspect and with one exception it has been prompt. We have further had great help in building up the infrastructure of our office in Baghdad and the field office in Mosul. Arrangements and services for our plane and our helicopters have been good. The environment has been workable."
> If I believed any conspiracy as far as Iraq goes
US disregarded that and still decided to invade. No one had a single doubt that this was about oil.
Iraq is a clear case of a conspiracy. This is actually a very good counter-point to conspiracy theorists: When a government tries to lie about something, it is really obvious, and it still works.