Blessing from the UN would have meant serious proofs that Saddam did constitute a threat to its neighbours.
For more than a decade, air above Iraq was controlled by the allied planes.
Many factories have been destroyed in 1991.
There was an embargo.
Spy satellites have orbited above Iraq countless of times.
UN inspectors have not found the smoking gun, even if Iraqis were wanting to reconstitute their arsenal, I haven't heard of anything WMD-ish coming up since the inspectors were allowed back or the US inspectors were free to check.
Saddam didn't invade any country since 1991.
All those hints didn't convince the UN.
War is far too serious a matter to be considered lightly and therefore an absolute unmistakable proof was needed.
I'm afraid the US administration doesn't have the same viewpoint of what a war can lead to as the rest of the world.
Perhaps, this will change in the coming months;

Raoul