Jonah Goldberg writes what I've long wondered at too:
For reasons that still baffle me, the WMD threat — never the sole reason to invade Iraq — not only became the only argument, it became a thoroughly legalistic one, as if foreign policy has rules of evidence and procedural due process...The fact that Hussein turned out to be bluffing about WMDs isn't a mark against Bush's decision. If you're a cop and a man pulls out a gun and points it at you, you're within your rights to shoot him, particularly if the man in question is a known criminal who's shot people before. If it turns out afterward that the gun wasn't loaded, that's not the cop's fault.
Hussein had a 30-year track record of pursuing WMDs. He dealt with Islamic terrorists. The sanctions regime fell apart thanks to Iraqi bribery and 30 years of spineless U.N. accommodation. In the 1990s, Hussein tried to kill a former U.S. president and tried to shoot down British and American planes enforcing the "no-fly" zone. The Clinton administration — not the George W. Bush administration — established "regime change" as our policy toward Iraq. In the years that followed, the Iraqi regime openly celebrated the 9/11 attack. And when we tried to get Hussein to come clean about a weapons program we (and his own generals!) had every reason to believe existed, he played games. After 9/11, calling that bluff wasn't a "choice," it was an obligation.