As Chilcot intimated, it wasn't a binary choice at that moment in March. Containment was still an option. Blair acknowledged that in the evidence. The US moved the goalposts and Blair with Straw acquiesced. See 508 in the summary.
Of course.
I think the motivation for this comes back to 9/11. I guess it's impossible to actually know, but I suspect that the slaughter there changed the perspective of the British government (on both sides of the house) to one which was much more likely to support their allies (the US) at all costs. Much more likely to blindly support them after the attack on them.
One area of the whole debacle is getting over simplified for me. The whole question of the "evidence" of WMDs was not a binary issue either. Intelligence like this is never a binary issue, and from what I've read, if it's 70/30 then you are doing extraordinarily well. The problem was (it seems) that he intelligence was more 51/49, and rather than present that clearly, it was made to seem much more certain.
Again though, I think we can see the shadow of 9/11 in the background. Intelligence there said there might be an attack, but it wasn't seen as terribly credible. They made the wrong judgement call then, and I wonder if this made the US/UK governments much more wary about ignoring threats which were much more of a 50:50 call.