Dr Hans Blix, former UN weapons inspector, has told the Chilcot Inquiry into the Iraq war that the US and UK didn't feel that they needed "a permission slip" from the security council:
"The US in 2002...I think they were high on military at the time. They said 'we can do it'
They thought they could get away with it so they decided to do it."
The lesson here is don't start wars when "high".