There is a telling statement in Tommy Franks' book where he comes out of a meeting at OSD and he and his Chief of Staff talk about how they dodged the bullet on PCR (Phase IV). Later Franks describes the message he sent to Wolfowitz where he said he would take care of the day of and Wolfowitz and Feith needed to take care of the day after.

Shinseki's "300,000" was a statement of the allied force needed to occupy the country. It was, at least in part, predicated on Jay Garner's planning for bringing back the Iraqi regular army which has been commented on elsewhere. The relevant point, IMO, is that the large numbers for occupation advocated by Shinseki could have been reduced in an orderly manner fairly quickly had the plan been implemented. But CENTCOM never took ownership of the plan crafted in Washington and neither did OSD. As a result, what Phase IV planning Franks did was clearly inadequate and ORHA planning was largely ignored by both CENTCOM & OSD.

Equally important, nobody except the OSD maligned GEN Shinseki seems to have given any thought to the number of troops needed for Phase IV.