I see Blair as the really evil bstard here. Bush was carried away by wanting to do something about 9/11 but did not know what or to whom; his redneck supporters demanded action. The others were in it for whatever tit-bits dropped off the table. Blair, however, schemed and plotted, twisted and lied, connived and compelled to advance his own cause. I think he saw himself as Thatcher re-born and that Iraq was to be his Falklands in future history.
Blair drove the process forward for the intervention in Serbia during 1999. Clinton was happy to offer up a few bombers, but little else. It set two clear precedences in the 'new' post-Cold War international system. It was Allied Force, its non-UNSC approval, and its 'humanitarian-just cause' notion that beat the path to the invasion of Iraq.
Striking Afghanistan to avenge the Twin Towers was not enough for the US, and they needed a more 'appropriate' target to mollify mid-Amerika. The Bush Administration settled on Iraq rather than Iran or Syria because they genuinely believed they had a 'good excuse': WMD. Of course, we ALL know in hindsight (some of us were inclined to believe before) that WMD didn't exist, and Iraq's only failure was to get its paperwork up to the impossible standard demanded by the US (via the UN). Chalabi, his INC, and others have spent many years spreading the intelligence that Iraq had WMD in order to get the mighty US military to topple Saddam for their own selfish gain. The Bush Administration saw what it wanted to see. However, unlike Serbia in 1999, Blair was only a bit-part player in this act who yhought it would be to the UK's benefit to ride the US's coat-tails.
Bush, Blair and Co will not be tried like the Bosnian/YU scumbags - but they should!