Who says the Pentagon did not have a plan for post-war Iraq? Everything appears to be proceeding swimmingly to plan - the plan to allow Iraq to disintegrate into civil war so to fragment the nation into a series of weaker states that individually, on the balance of probabilities, will be easier to influence and control.
Iraq's autocratic rulers, the Bathists under Saddam Hussein, were emboldened, by the wealth of oil and autonomous power, to resist the coercions of foreign powers. Having previously succumbed, in their fruitless war with Iran, they learnt to resist the hand of their master with which they used to be fed.
The frustration of such indiscipline lead those who thought themselves to still be Iraq's masters to beat and eventually destroy their once useful hound. And to ensure that Iraq would not resurrect, phantom like, it's corpse was to be dismembered. Should one of the new smaller Iraqi states fail to tow the line (as is probable) it's power and strategic regional threat posed (to you know who) will be much diminished from that considered to exist with Saddam's Iraq and mixing it up yet again, in the hope of a preferable fresh outcome, an easier task.
As Henry Ford said "Nothing is particularly hard if you divide it into small jobs"