Well, at 45% off at Sam's Club, I had to buy "Cobra II", the inside story of the invasion of Iraq. My impression from the first few chapters is this: Former corporate exec comes in as Sec. of Defense. Sees military as bloated and overly bureaucratic. Tired of huge armies that take forever to mobilize, ala Gulf War. Long mobilization and large troop levels eliminates all element of surprise and antagonizes allies and their restless citizenries. Sec. of Def. sees Afghanistan war of example of success. Negotiates with Tommy Franks over troop levels, arguing him down from near 400K to initial 150K. Can win war with those levels but not win peace. Doesn't think about occupation because idea was there would be no occupation. Get the hell out of Dodge!
One line from the book perfectly expressed my pre-war thoughts: "While British officers were worried about the state of the U.S. planning, the civilians in Blair's cabinent were more assured. It was a faith-based confidence grounded on the assurance that Bush wold not stake his presidency on a war with Iraq unless he knew what he was doing." In other words, no one could have a blind spot that big. Securing the Iraqi border alone would take a lot of troops, wouldn't it? But Rumsfeld was an "enthusiast", a reformer, and reformers often make mistakes. As a CEO, you often can't achieve change without the hubris of thinking yourself more knowledgeable than your subordinates, and yet that very hubris is the cause of many a downfall. A catch-22.
So the short answer is they thought there'd be no occupation. And why? Because sometimes when you desire something very much - such as Rumfeld's desire for a modern, more nimble army - blinded him to realities.