My response to Ricks:
The 'mistakes' of flawed military adventures by empires by and large remain venial if lessons available to be learned are learned - they become mortal when such lessons along with incumbent adaptations are lost for whatever reason. Since it's illogical to assume that everything an empire does in pursuit of its needs is sensible or justifiable a priori it follows that 'mistakes' are not only inevitable but necessary and that the Iraq adventure therefore can only be designated the 'biggest mistake ever' if inherent lessons have not been learned - but your book's thesis is that lessons have been learned and put into practice thus rendering your sweeping characterization of the war somewhat nonsensical .
It's really about time that critics stop analyzing the war completely in the shadow of how it was managed - there's a broader context here that has very little to do with the competence of the former Commander in Chief. For instance you casually imply in your post that one of the reasons Iraq was 'the biggest mistake ever' was that we were already at war somewhere else, a complaint to which the reasonable response is: one, our troop commitment in Afghanistan was small; and two, our whole military is, or at least was premised on the ability to conduct simultaneous actions. Your criticism is therefore narrow to a fault and absurdly presupposes a quite obviously flawed postulate that empires don't make mistakes!
Part of a more rational approach to understanding the war would be to say it revealed how many false assumptions were underwriting our strategy - the important point being that we were fated to learn that lesson and possibly that it's for the best we learned it now instead of at some future point that may have proved less forgiving [Iran?]. Therefore, within that context, from that broader, historical view, one can with credibility speculate that continuing attempts to characterize Iraq as 'the biggest mistake ever' et al are classic examples of not seeing forest because of trees - speculate that possibly the most dire and dangerous consequences stem not from the war per se but from the litany of false impressions stirred up and left to linger in its wake.
This is not to absurdly defend any actions an empire might take but rather to defend a moderating reality, often missed by lobbyists for and against, that implies there is a dynamic in which an empire must move in order to remain viable the benefits of which cannot always immediately be known or reduced to a balance sheet that clearly states what is lost and what is gained.