IMO the truth of the matter is, that after 9/11, the US, and Mr Bush and friends in particular,simply had to do something to show that they were still "kings of the shitheap".....mainly for domestic consumption.
They chose a weakened(by sanctions) country, run by a troublesome and unpopular ex friend and dictator(Saddam), which they presumed would be an easy conquest militarily and which the rest of the world would not be to perturbed about.

In fact, the infrastructure of Iraq and the Iraqi people were to be sacrificed to restore America's pride.

Blair became convinced of the plan and saw reflected glory in becoming joined at the shoulder to the Commander in chief.
The UK cabinet which contained so called "lefties" demonstrated their cowardice and hypocricy by ditching their credentials and allowing Blair to set the agenda, because he was an electible leader and could, with a good result in Iraq, could guarentee their jobs for life.
This with one exception, Mr Robin Cook, who was shrewd enough to foresee that they were sleepwalking into a political and military nightmare, but unfortunately not sufficiently psycic to foesee his own demise.
Mr Cook resigned from the cabinet, I believe with the intention of returning as the saviour of Labour and Labour ideals, when the whole enterprise ended in tears and recriminations.

Mr Cook died shortly after, adding credence to the auld quote by Rabbie Burns.....that "the best laid plans o' rats an' politicians gie often come aff!"

Blair and Bush "marched into Baghdad tae the beat o' the drum,
                tae be showered wi' bananas and Pedigree Chum"(dog food)................from"The chimp and the poodle" by Davie R.

The rest, as they say ...is history....Ake