They are discussing pacification stabilization worst-case scenario not World War Three.
Once Saddam is disposed there will be a power vacuum.
The worst case scenario is that splinter groups will rise up demanding the right to secede from Iraq Proper.
Other groups will start fighting for the Iraqi leadership.
Such a scenario will mean the placing of large numbers of Troops mainly Americans in a pacification stabilization role.
The longer American troops are based there as peace keepers the more embroiled they would become in local politics/grievance, the more of a target they will become from breakaway splinter groups, pan Arab Nationalists and Islamic fundamentalists.
Tony