Perhaps if Saddam was successfully removed from power and is trapped in Baghdad surrounded by US forces.

1) Saddam is a madman, a fucking psycho, he will be bound to be trigger happy and use his chemical weapons on anyone, even his own people if he ever gets surrounded. They wil be high casualities for both the US and normal civilians.

2)If Saddam is removed there could possibly be a power vacuam in Iraq and instability. Those Chemical weapons can get into the hands of renegade generals who would sell them to terrorist or simply just give them away for free. Either way the selling and the arming of terrorist with weapons of mass destruction cannot be prevented, they are too well hidden and Iraq is just so darn big.

3)Saddam may be a madman but he aint no fool. As soon as he realizes he wil lose those weapons of mass destruction will be quickly given away most likely or used. Also the fact that he will burn every single gallon of oil in Iraq.

4)More recruits for Al Queda and the birth of more terrorist cells with huge support from the population of the Middle-East.