I'd say the answer to all of the above is UN Resolution 1441. It promised "serious consequences" for not abiding. It was never up to the UN to prove Saddam had destroyed WMD, in fact, it was the other way around. Which he could not do. There was not to be a trial, it was held and he was guilty. He simply was put on probation and failed to show up for his appointments. Simplistic way of looking at it but that is what it is in a nutshell. He simply was in material breach of the resolution and his own peace accord after GW1.
It is not the fault of the US that the UN would not follow up on it's threats. It is not the fault that most of the UNSC were bigtime arms dealers to Iraq. So to answer the question, when it came to Iraq, the UN itself despite it's immasculation set the tone. It just couldn't find its way out of the national interests of the UNSC members to follow thru.
Before anyone says anything about not finding it, it doesn't matter. THe resolution provided that Saddam show proof he did not have it. He did not and up to now they still haven't been able to. The very idea that anyone in the west would believe anything coming from him as proof is absurd.
As for the sure argument of "what does serious consequences mean?" I'd say that without a doubt it means that military measures would be undertaken. They could not do anymore damage to Iraq economically nor politically. Military means were all that was left. I contend that the US thru resolution 1441 were given the authority to invade Iraq and overthrow the regime there. Just because the UN won't follow up on it's own resolutions does not throw the blame on this nation.
Here's a good reason why some may not have wanted to do anything against the guy.