You forget that Gulf War I never really "ended," per se. It ended with a ceasefire and thus such infractions warrant a different course of action given their previous hostility towards and ally and your own troops.
As Cheney pointed out last night, Iraq was a nexus for terrorists and a likely place to trade WMDs.
And if Saddam was so law-abiding, why didn't he let the inspectors in and show proof?
Why would he have paid off the UN to trade illegally through Oil for Food?
Why would he have sold his oil cheaply if he didn't have to? Europe got very cheap oil from him.
How could he have racked up billions in personal assets, billions for payoffs and billions in Military debts to Russia, France and China (and others) when the oil industry was only worth $100million (total, not annual)?