Ask yourself the following question. Considering the US administration's determination to go to war no matter what.
Did Blair's decision increase the number of deaths in Iraq?
No. In fact it might even have reduced deaths by our moderating influence on the US.
Whatever you think about the justification for the war or not (I'm in the camp who was against the war), we must remember that Blair's decision was irrelevant to whether the war went ahead or not.