gah.
it just doesn't work like that, having some cutoff which is too many people dying and suddenly it's not worth it anymore. we set a national objective of removing saddam from power for lots of reasons, most of which can be traced back to our own national security (and i'm not talking about WMDs)
when that happens and you go after it, you go after it 100% and some people are going to die. we could have removed saddam from power by flattening their country, but that wouldn't be very nice of us. war is a messy messy business, especially when you're playing by the rules and the bad guys aren't