The biggest mistake was that we did not crush Iraq sufficiently in the original conflict. We didn't defeat Iraq's army we accomplished a coup d'etat by just entering Baghdad and saying 'here we are'.
The armchair generals say we should have kept the Iraqi army together, but we didn't disband their army they just melted back into the population.
Well, let's just say for the sake of argument that you're right even though you're not, precisely how would you propose that we should have crushed Iraq more sufficiently than we did in actuality?