Consistent with military and strategic logic, and with leaked Pentagon war plans from the summer of 2002, I'm assuming that a war to overthrow Saddam would involve about 250,000 American forces. The Afghanistan model of warfare, in which small numbers of U.S. special forces and American airpower work with indigenous opposition groups to fight government forces, would almost surely not work in Iraq. That is due to relative weakness of the Iraqi opposition as well as the Iraqi military's ability to hole up in cities, where American airpower is far less effective than in open terrain. Modest-sized operations, involving perhaps 50,000 to 75,000 U.S. troops, are somewhat more promising. But they would run the risk of encountering serious difficulties in the urban centers of Iraq. Additionally, Iraqi forces would be less likely to capitulate quickly if they sensed they had a chance to prevail, increasing the odds of a prolonged urban battle under such circumstances.
This is not to say that a larger operation would have to mirror Desert Storm in its basic concept. The invasion might involve rapid airborne or commando strikes against Iraqi command and control assets as well as weapons of mass destruction sites in the earliest hours of combat, even as main invasion forces march more slowly through Iraq toward Baghdad and other cities. It might also use relatively small teams of American ground forces—perhaps brigade-sized units of several thousand troops each—to try to lure Iraqi forces out of the cities into regions where they would be more vulnerable to American airpower (and to lure out defectors to join U.S. forces). These sorts of "inside-out" tactics would try to avoid the delays inherent in a mechanized march from Kuwait and other neighboring countries to Baghdad. But they would be gambles, and the United States would need backup forces in place in case the gambles did not pay off.