When Mr Straw's own legal adviser told him an invasion without a second UN resolution would amount to a crime of aggression, he rejected the advice.

(Jack Straw) My worry about this was that if we didn’t get a second resolution and didn’t pursue the matter, that the whole effort would be diminished and degraded and we would be back in a worse position with Saddam than we thought, and bear in mind behind all of this was our profound belief that Saddam had, and was going to use, weapons of mass destruction. If I may say, Sir John, so profound that in the Foreign Office we acquired for our staff protective clothing for them, including protective gas masks, which I actually had in my office and inspected myself.