Donald Rumsfeld likes making lists. This is a man, after all, who lives by a collection of maxims known as "Rumsfeld's Rules." Yet few lists the defense secretary has ever compiled are more ominous than the one that now sits on his desk at the Pentagon. It is a collection of things that could go wrong if the United States goes to war with Iraq, and for months he has been steadily adding to it. He has yet to cross anything off.
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Yet beneath the confidence among U.S. officials about the outcome, a general unease exists about the unintended consequences of trying to take down Saddam Hussein's regime. It could go smoothly: Michael O'Hanlon of the Brookings Institution estimates that as few as 100 U.S. soldiers may be killed. If things go badly, he predicts, that figure could hit 5,000. Saddam, many fear, like the biblical Samson, will bring the walls of the temple down around himself. "Based on a fair amount of trying to figure Saddam and his cronies out, I wouldn't try to predict how they will behave," remarks one senior Pentagon planner. "That's what makes them so dangerous." The following are scenarios that war planners tell U.S. News keep them up at night. Some of their worst-case scenarios they refuse to divulge, for fear of giving Saddam any more ideas.

1. Iraqi forces unleash their chemical or biological weapons arsenal.
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2. Saddam Hussein makes a bloody last stand in Baghdad.
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3. Iraq's oil wells are turned into fields of fire.
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4. Saddam puts civilians in harm's way.
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5. Terrorists acquire Saddam's weapons of mass destruction.
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6. Once Saddam is ousted, Iraq descends into chaos.

» U.S. News: Six deadly fears: The U.S. military is confident of victory in Iraq--but at what price?